How to Make It
In a mini food processor, sprinkle the diced bread with the water and let stand until the water is absorbed, about 5 minutes. Add the garlic, cayenne and salt and process until the bread and garlic are coarsely chopped. With the machine on, drizzle in the olive oil and process until the rouille is smooth. Transfer to a bowl and refrigerate.
In a very large, deep skillet, heat the 3 tablespoons of olive oil. Add the leeks, onion, fennel and chopped garlic and cook over moderate heat until translucent, about 5 minutes. Add the tomatoes and cook until they begin to break down, about 5 minutes. Add the bay leaves, saffron and pastis and bring to a boil. Add the fish stock and bring to a simmer. Cook over low heat until the vegetables are very tender, about 20 minutes. Discard the bay leaves.
In a food processor, pulse the vegetables and broth to a coarse puree. Strain through a fine sieve set over the skillet.
Bring a large pot of water to a boil. Add the lobster and cook until it turns bright red, about 4 minutes. Drain and rinse the lobster under cold water until cool enough to handle. Remove the tail, claw and knuckle meat and cut into 1-inch pieces.
Preheat the broiler. Arrange the baguette slices on a baking sheet and broil them 6 inches from the heat for about 1 minute per side, until the slices are golden brown around the edges. Rub each slice with the remaining whole garlic clove and drizzle lightly with olive oil.
Add the potatoes and cayenne pepper to the broth and bring to a simmer. Cook over moderately high heat until the potatoes are just tender, about 10 minutes; season with salt and pepper. Add the clams, cover and cook over moderate heat until they just begin to open, about 3 minutes. Add the monkfish, cover and simmer for 2 minutes. Add the lobster, snapper and halibut, cover and simmer until the clams are open and all the fish is cooked through, about 4 minutes.
Set a baguette toast in each of 8 shallow bowls. Ladle the fish and broth over the toasts and top each serving with 1 tablespoon of the rouille. Sprinkle with fennel fronds and serve immediately.