Fettuccine ai Frutti di Mare

Name: Fettuccine ai Frutti di Mare
Ingredients: 3 tablespoons olive oil; 1 large onion, chopped; 4 garlic cloves, crushed; Kosher salt, freshly ground pepper; 2 oil-packed anchovy fillets, chopped; 1 tablespoon tomato paste; 5 cups canned diced tomatoes, plus ½ cup tomato juice; 3 tablespoons chopped basil; ¼ teaspoon red pepper flakes; ½ cup clam juice; ½ cup dry white wine; 12 small clams, such as Manila or littleneck, scrubbed; 16 mussels, scrubbed, debearded; 10 ounces large shrimp, peeled, deveined; 10 ounces bay scallops; 12 ounces fresh fettuccine (or 8 ounces dried); 5 ounces baby spinach leaves