Step 1

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.

Step 2

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.

Step 3

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.

Step 4

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

Step 5

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.

Step 6

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.

Step 7

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.

Step 8

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