Preheat oven to 400°. Cut off top of garlic to expose a bit of each clove. Put on a piece of foil, drizzle with 2 tsp. oil, and wrap foil to seal. Roast until garlic softens and starts to brown, 35 to 40 minutes.
Put chiles, meanwhile, in a bowl. Add 2 cups boiling water and let stand until softened, 30 minutes. Drain chiles and scrape out seeds.
Wipe grill grates with oiled paper towels. Heat grill to medium (350° to 450°). Whirl almonds and bread in a food processor until coarsely ground. Pushing from the bottom, squeeze garlic out of skins and into bowl. Add red peppers, chiles, remaining oil, tomatoes, vinegar, honey, and salt. Pulse romesco until smooth. Makes 3 1/3 cups sauce.
Rinse fish and pat dry. Using a sharp knife, cut a few slashes into the sides, parallel to the gills. Rub fish liberally inside and out with romesco sauce.
Grill fish, covered, until starting to brown, 4 to 6 minutes. Using two large metal spatulas, carefully turn fish over once. Cook fish, brushing a little more sauce on it as you go, about 5 to 6 more minutes. Insert a thin knife or metal skewer into the thickest part of the fish and leave it for 30 seconds. Remove skewer and hold it against the inside of your wrist; it should feel warm. If it's still cool, grill fish another 2 to 5 minutes. Carefully transfer fish to a platter. Serve extra sauce on the side.
*Guajillo chiles are relatively spicy and common in Mexican cooking; find them at well-stocked grocery stores or Latino markets.
Note: Nutritional analysis is per serving with 1/4 cup sauce.