Heat olive oil over medium heat in a 7-quart Dutch oven until shimmering. Add onion, leek, fennel, garlic, fennel seeds, saffron, orange zest, cayenne pepper, and thyme sprigs. Cook, stirring occasionally, until vegetables have softened, about 10 minutes.

Stir in tomato paste and cook for 2 minutes. Add diced tomatoes and cook, stirring, until tomatoes begin to soften, about 3 minutes.

Layer in whole fish and/or fish bones and heads, stirring to mix.

Add white wine and Pernod or pastis (if using), stirring to scrape up any bits from the bottom of the pot. Bring to a boil over medium-high heat, then boil until raw alcohol smell has cooked off, about 3 minutes.

Add enough boiling water to fully cover all ingredients in the pot. Add parsley and bay leaf, increase heat to high, and bring broth to a vigorous boil. Allow to boil rapidly for 5 minutes. Lower heat to a simmer and continue to cook until broth is flavorful, about 45 minutes. Season with salt and pepper.

Process garlic, bread, cayenne pepper, saffron, and egg yolk to a paste. Work in 2 tablespoons liquid from fish broth pot to thin paste slightly. Season with salt.

Drizzle in olive oil while mixing. Add fish broth, 1 tablespoon at a time, until thick, mayonnaise-like consistency is reached. Refrigerate until ready to use, up to 3 days.

Transfer broth and all ingredients to a blender. Blend until as smooth as possible.

Strain broth through a fine-mesh strainer, season with salt and pepper.

Return broth to heat and gently poach seasoned fish until just cooked through.

If using mussels or crabs, add to gently simmering broth and cook until done.

Serve broth and fish with baguette toasts and rouille.