Put oven rack in middle position and preheat oven to 250°F.

Arrange bread slices in 1 layer in a shallow baking pan and brush both sides with oil. Bake until crisp, about 30 minutes. Rub 1 side of each toast with a cut side of garlic.

Plunge lobster headfirst into a 6- to 8-quart pot of boiling water, then cook, covered, 2 minutes from time lobster enters water. Transfer lobster with tongs to a colander and let stand until cool enough to handle. Discard hot water in pot. Put lobster in a shallow baking pan. Twist off claws with knuckles from body, then crack claws with a mallet or rolling pin and separate claws from knuckles. Halve body and tail lengthwise through shell with kitchen shears, then cut crosswise through shell into 2-inch pieces. Reserve lobster juices that accumulate in baking pan.

Cook tomatoes, onion, and garlic in oil in cleaned 6- to 8-quart pot over moderate heat, stirring occasionally, until onion is softened, 5 to 7 minutes. Meanwhile, peel potatoes and cut into ½-inch cubes. Stir potatoes into tomatoes with fennel fronds, bay leaf, saffron, sea salt, and pepper. Add stock and bring to a boil, then reduce heat and simmer, covered, until potatoes are almost tender, 8 to 10 minutes.

Add thicker pieces of fish and cockles to soup and simmer, covered, 2 minutes. Stir in mussels, shrimp, lobster, including juices, and remaining fish and simmer, covered, until they are just cooked through and mussels open wide, about 5 minutes.

Stir 3 tablespoons broth from soup into rouille until blended.

Arrange 2 croutons in each of 6 to 8 deep soup bowls. Carefully transfer fish and shellfish from soup to croutons with a slotted spoon, then ladle some broth with vegetables over seafood.

Top each serving with 1 teaspoon rouille and serve remainder on the side.