Sometimes a sauce is more than, well, just a sauce. Discovered for the first time — on the menu of a restaurant, amid the pages of a cookbook — it looks ordinary enough. But in one bite such a sauce transforms the dish, then the meal, then the diner.

If you think I’m overstating then you’ve never experienced a good romesco sauce.

This classic Catalonian sauce –– thick as pesto, the color of rust, textured with nuts and a bit of fried bread — packs an astonishing amount of flavor into small acreage. Earthy, toothsome, definitely habit-forming, romesco is rough magic in a bowl.

It’s also surprising, as nut sauces can often be, because at first you can’t quite place the earthy undertones and complex textures. High notes of sherry and paprika yield to the deep flavors of hazelnuts and almonds; sweet octaves of tomato and pepper follow next, then there’s an aftershock of garlic, maybe another of chile.

Make a bowl of it, a very large bowl. Then scoop up the sauce with a slice of toasted bread, a shrimp from the grill, a sweet spring onion slipped from the charred filigree of its skin.

In Catalonia cooking, romesco is stirred into seafood stews, spooned over fish and served in bowls as a condiment.

Because it’s a sauce built with healthful nuts instead of, say, butter, a romesco is more than just an aesthetic addition or a flavor boost to a dish. Paired with bread or onions, it can be a satisfying course unto itself. You can also ladle it into soups or over spring lamb, fish kebabs or grilled vegetables. Eat it out of the bowl with a spoon.

Romesco isn’t a hot sauce. It’s a subtle cohesion of flavors and textures, with a little sweet heat and depth but nothing overpowering. The nuts form the base of the sauce and give it a slightly rough texture. And the bread isn’t a thrifty cook’s filler, but a way to smooth and balance the nuts and add body. A mild bite comes more from the garlic and vinegar and paprika than the peppers. The finely balanced, intricate notes of nut and spice underscore delicate flavors — briny seafood, sweet onions — without overwhelming them, providing a little punch rather than a knockout.

That’s why in Spain the sauce, in a regional variation called a “salbitxada,” is paired with the grilled spring onions called “calcots.” It’s the featured sauce in the spring onion festival — the “calcotada” — where the delicate onions are grilled, then dipped in bowls of the heady stuff.

Traditionally, romesco is made with dried Nora peppers, a Spanish variety that’s a visual dead ringer for the Cascabel chile pepper, a Mexican pepper named for the drum rattle it resembles. The Nora has the same color and earthy notes as the Cascabel but is sweeter.

Noras are available in stores supplied by Spanish importers or online. But since the peppers weren’t always so easy to find outside Spain, many cooks have improvised, using roasted red peppers, a bigger hit of paprika or replacing the Noras with ancho peppers or something even hotter. As with any good sauce, variants abound.

At Bar Pintxo in Santa Monica, Calif., chef-owner Joe Miller throws a few pickled Spanish green Guindilla peppers into his romesco. Mozza executive chef Matt Molina does his with earthy smoked paprika.

La Espanola co-owner Juana Faraone has a copy of a 50-year-old Catalonian recipe that, along with the Noras, includes roasted onions.

If you don’t want to use Noras, or if you want a sauce with more heat, you can experiment with the many dried peppers available. Many cooks like making romesco with anchos, but I like the verisimilitude of using Cascabels. Blend in a pair along with a charred red bell pepper and a little parsley and you’ll have a romesco with more depth and bite than a traditional version.

A classic version, with more finesse and less heat, showcases the delicate notes of the nuts and a sweet Spanish paprika. For an additional touch of refinement, use plump Marcona almonds with the hazelnuts — their delicate, buttery flavor comes through in the milder sauce.

And since it’s spring onion season, now is the perfect time for your own calcotada. Roast a bunch whole, throw thick slices of bread and plump shrimp on the grill too, and eat them all with a bowlful of romesco — and your hands.

Some sauces are not made for dainty dipping but for palpable, messy, unapologetic pleasure.

ROMESCO WITH GRILLED BREAD, SPRING ONIONS AND SHRIMP1

ROMESCO WITH GRILLED BREAD, SPRING ONIONS AND SHRIMP

1 pound tail-on large shrimp, peeled and deveined

2 bunches large spring onions, bulbs trimmed of their roots and greens attached

Best-quality olive oil, preferably Spanish

1 loaf pain rustique or country white bread, sliced 3/4-inch thick on the diagonal

Sea salt

Romesco Sauce (see recipe)

Heat a grill over medium heat. Toss the shrimp and the onions with a little olive oil and a sprinkling of salt; brush the bread with olive oil and sprinkle with salt.

Grill the onions until the bulbs are tender and with good grill marks and the green parts are somewhat charred, several minutes. Rotate the onions occasionally with tongs for even grilling. Set aside.

Clean the surface of the grill and cook the shrimp next, rotating for even grilling just until the flesh is opaque and firm.

Clean the surface of the grill and grill the bread, rotating for even grilling. Serve the warm bread, shrimp and onions on a platter with a large bowl of romesco sauce in the center for dipping.

Servings: 4 to 8

Each of 8 servings: 433 calories; 17 grams protein; 37 g carbohydrates; 4 g fiber; 25 g fat; 3 g saturated fat; 84 milligrams cholesterol; 748 mg sodium.

BASIC ROMESCO SAUCE1

BASIC ROMESCO SAUCE

8 to 10 Nora peppers (see note)

1/4 cup hazelnuts

1/4 cup Marcona almonds

4 Roma tomatoes

1/4 cup plus 2 tablespoons best-quality olive oil, divided

1 ounce good-quality country white bread, sliced, crusts removed

3 cloves garlic

1 teaspoon sea salt

1 teaspoon sweet paprika (preferably Spanish)

2 tablespoons sherry vinegar

Place a rack in the upper third of the oven and heat the oven to 375 degrees. Cover the peppers with boiling water and allow to soften for at least 30 minutes, then stem, seed and reserve.

While the peppers are softening, toast the nuts separately until golden and aromatic, 8 to 10 minutes. If the hazelnuts have skins, remove their skins by rolling them in a kitchen towel once they are cool. Set the nuts aside and raise the oven temperature to broil.

Halve the tomatoes lengthwise and place them, skin side up, on a foil-lined baking sheet. Brush 1 tablespoon of oil over the tomatoes and broil the tomatoes until the skins begin to darken and crack, about 5 minutes. Cool on their tray, then peel, core and set aside.

In a skillet, heat 1 tablespoon of oil and fry the bread until golden brown. Cool and set aside.

In a food processor, coarsely chop the garlic, salt, fried bread and nuts. Add the peppers, tomatoes, paprika and vinegar and process to a rough paste. Slowly pour in the remaining olive oil in a steady stream and process until just combined.

Note: Nora peppers are available from Spanish importers online such as laespanolameats.com andtienda.com, as well as amazon.com. You can substitute Cascabels for the Nora peppers (they’re hotter than Noras).

Makes about 2 cups.

Each tablespoon: 40 calories; 1 gram protein; 1 g carbohydrate; 0 fiber; 4 g fat; 0 saturated fat; 0 cholesterol; 78 milligrams sodium.

ROASTED RED PEPPER-CASCABEL ROMESCO SAUCE1

ROASTED RED PEPPER-CASCABEL ROMESCO SAUCE

2 dried Cascabel peppers

1 large red bell pepper

1/4 cup hazelnuts

1/4 cup almonds, blanched and peeled

4 Roma tomatoes

1/4 cup plus 2 tablespoons extra virgin olive oil, divided

1 ounce good-quality country white bread, sliced, crusts removed

3 cloves garlic

1 teaspoon sea salt

1 teaspoon sweet paprika (preferably Spanish)

2 tablespoons sherry vinegar

1 tablespoon minced flat-leaf parsley

Place a rack in the upper third of the oven and heat the oven to 375 degrees. Stick a fork through the Cascabels and place tines-down in a bowl to keep the peppers submerged. Cover with boiling water for at least 30 minutes to soften, then stem, seed and set aside.

While the Cascabels are softening, roast the red pepper on a gas stove or under a broiler. Place the peppers in a plastic bag or in a bowl, covered with plastic wrap, and cool. Peel, stem and seed the peppers (don’t rinse); set aside.

Toast the nuts separately in the 375-degree oven until golden and aromatic, 8 to 10 minutes. If the hazelnuts have skins, cool them and remove their skins by rolling them in a kitchen towel. Set aside the nuts and increase the oven temperature to broil.

Halve the tomatoes lengthwise and place them, skin side up, on a foil-lined baking sheet. Coat the tomatoes with 1 tablespoon of olive oil, then broil the tomatoes until the skins begin to darken and crack, about 5 minutes. Cool on the baking sheet, then peel, core and set aside.

In a skillet, heat the remaining tablespoon of olive oil and fry the bread until golden brown. Cool and set aside.

In a food processor, coarsely chop the garlic, salt, fried bread and nuts. Add the peppers, tomatoes, paprika, vinegar and parsley and process to a rough paste. Slowly pour the remaining olive oil in a steady stream and process until combined.

Makes about 2 cups.

Each tablespoon: 44 calories; 1 gram protein; 2 g carbohydrates; 1 g fiber; 4 g fat; 0 saturated fat; 0 cholesterol; 79 milligrams sodium.