Fettuccine ai Frutti di Mare

Heat oil in a large saucepan over medium. Cook onion and garlic, stirring, until onions are translucent, 5–8 minutes; season with salt and pepper. Add anchovies and tomato paste and cook, stirring, until color deepens, about 2 minutes.

Stir in tomatoes, tomato juice, basil, red pepper flakes, and 2 tsp. salt. Bring to a boil, then reduce heat and simmer until sauce is thickened, about 15 minutes.

Add clam juice and wine and bring to a boil; cook 2 minutes. Lower heat and bring to a rapid simmer. Add clams and cook until they open, 8–10 minutes. Transfer clams to a large warm bowl; discard any that do not open.

Add mussels and cook until they open, 3–5 minutes. Transfer mussels to bowl with clams; discard any that do not open.

Return sauce to a boil, add shrimp and scallops, cover, and remove from heat. Let sit until shrimp and scallops are cooked through, 5–8 minutes. Transfer shrimp and scallops to another large warm bowl.

Meanwhile, cook fettuccine in a large pot of boiling salted water until barely al dente, about 2 minutes if using fresh pasta, reserving 2 Tbsp. pasta cooking liquid.

Return sauce to a boil, add spinach, and cook until spinach is wilted, about 1 minute. Add pasta and pasta cooking liquid; simmer until sauce thickens, 2–5 minutes. Add shrimp and scallops and continue cooking until well combined, 30–60 seconds. Transfer pasta mixture to a large bowl; season with salt and pepper. Arrange clams and mussels over pasta.

Do Ahead: Sauce can be made 2 days ahead. Let cool, store in an airtight container, and chill, or freeze up to 3 months.