• 2 lb. cod fillets, cut into 1-in. pieces
Heat oil in a large Dutch oven or soup pot over medium heat. Add fennel, leek and onion. Cook and stir on medium-low until softened, about 10 minutes. Stir in garlic and cook a couple of minutes. Do not brown the vegetables.
Add the tomato purée, seafood stock, vermouth, 1/4 cup water, saffron and cayenne pepper. Heat to a boil; reduce heat to low, and simmer uncovered, stirring often, about 20 minutes. Purée smooth with immersion blender or in a blender, working carefully in small batches. (The base can be made several days in advance; refrigerate covered.)
Reheat the base, and adjust with up to 1 cup water if the soup is thicker than heavy cream. Stir in 1/2 cup of the rouille. Season with salt (about 1 teaspoon) and a generous 1/4 teaspoon pepper. Add the fish. Simmer until fish flakes easily with a fork, about 10 minutes. Use a fork to break up the fish. Season again with more salt and pepper as needed.
Serve with remaining rouille and toasted bread.