Romesco sauce, a piquant mixture of chilies, nuts, and tomatoes, is a typical seasoning for seafood in the Catalan province of Tarragona. Although fresh coriander is not an ingredient found in the traditional Spanish pantry, its flavor lends a nice touch to this dish.

For sauce
2 dried mild red chilies such as New Mexico*, seeded and torn into large pieces (wear rubber gloves)
1/3 cup red-wine vinegar
1/4 cup whole blanched almonds
2/3 cup olive oil
2 slices firm white sandwich bread
1 medium onion, chopped
3 large garlic cloves, chopped coarse
4 plum tomatoes (about 1/2 pound), peeled, seeded, and chopped
1 teaspoon sweet paprika

1/2 cup dry white wine
1/2 cup water
2 pounds mussels (preferably cultivated), scrubbed well and beards pulled off
2 tablespoons finely chopped, washed-well, and spun-dry fresh coriander sprigs

*available at Hispanic markets and some specialty foods shops

Make sauce:
In a small bowl soak chilies, flesh sides down, in vinegar 30 minutes. Remove chilies, reserving vinegar in bowl, and scrape flesh from skins of chilies, discarding skins. Return chili flesh to reserved vinegar.

In a heavy skillet cook almonds in 2 tablespoons oil over moderately low heat, stirring, until golden and transfer with a slotted spoon to paper towels to drain. Heat oil remaining in skillet over moderate heat until hot but not smoking and fry bread, turning it, until golden. Transfer bread to a cutting board and cut into cubes.

In a skillet heat remaining oil over moderate heat until hot but not smoking and cook onion, stirring, until edges are just golden. Add garlic and cook, stirring, until garlic is pale golden. Add chili mixture, almonds, bread cubes, tomatoes, and paprika and cook, stirring occasionally, 2 minutes. In a food processor purée mixture coarse and season with salt. Sauce may be made 1 week ahead and chilled, covered.

In a heavy kettle stir together wine, water, and 1 cup sauce, reserving remaining sauce for another use, and bring to a boil. Add mussels and boil gently, covered, 4 minutes, or until most are opened.

Transfer mussels to a serving dish, discarding any unopened ones. Simmer sauce, stirring, until reduced and thickened slightly and spoon over mussels. Sprinkle mussels with coriander.