How to Make It
Preheat oven to 425°F. Cut tomatoes in half, and grate, flesh sides 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 1 cup onion.) Set aside.
Heat a 16-inch paella pan over medium-high; add safflower oil. When oil is hot, add shrimp. Cook shrimp until seared on both sides, about 1 minute per side. Transfer shrimp to a plate. Add 3 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. Add 2 tablespoons olive oil to pan, 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 pan. Cook until liquid evaporates and tomato pulp is deeply browned, about 12 minutes. Stir in pimentón; cook, stirring constantly, just until aromatic, about 30 seconds. Stir in 1 cup seafood stock, scraping up any browned bits from bottom of pan. Add remaining 2 1/2 cups 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 pan, and stir to distribute evenly. Transfer pan to preheated oven.
Bake fideuà 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 pan and top 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.