How to Make It
Preheat oven to 425°F. Cut tomatoes in half, and grate, flesh side down, on large holes of a box grater until only skin remains. Discard skins; set tomato pulp aside. Grate onion on large holes of box grater; finely mince any remaining large pieces. (You should have about 3/4 cup onion.) Set aside.
Heat a 12-inch oven-safe stainless-steel skillet over medium-high, and add safflower oil. When oil is hot, add shrimp. Cook until seared, about 1 minute per side. Transfer shrimp to a plate. Add 2 tablespoons extra-virgin olive oil; stir in pasta. Cook, stirring often, until pasta is toasted and darkened, 5 to 7 minutes. Transfer to a plate. Reduce heat to medium. Add 3 tablespoons extra-virgin olive oil to skillet, and stir in grated onion. Cook, stirring often, until golden brown, about 7 minutes. Add garlic, salt, and remaining 1 tablespoon olive oil. Cook, stirring often, until onion and garlic are deep brown, about 4 minutes. Stir in tomato pulp, scraping up any brown bits on bottom of skillet, and cook until liquid evaporates and tomato pulp is deeply browned, about 18 minutes. Stir in pimenton, and cook, stirring constantly, just until aromatic, about 30 seconds. Stir in 1 cup seafood stock, scraping up any browned bits from bottom of skillet. Add remaining 1 cup stock.
Increase heat to high, and bring to a boil. Reduce heat to medium, and simmer 10 minutes. Add salt to taste. Return pasta to skillet, and stir to distribute evenly. Transfer skillet to preheated oven.
Bake 5 minutes; remove from oven, and arrange clams and mussels in a single layer on top. Return to oven, and bake until a crust has formed around edges of skillet and top of pasta is lightly toasted, about 15 minutes.
Remove from oven, and discard any unopened clams and mussels. Arrange shrimp over top, cover with aluminum foil, and let stand 5 minutes. Serve with aioli and lemon wedges.