In a 7-quart Dutch oven, heat olive oil over medium heat 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; lower heat if necessary to prevent browning.

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

Using a mortar and pestle, a blender, or a food processor, process garlic, bread, cayenne pepper, saffron, and egg yolk to a paste. Work in 2 tablespoons (30ml) liquid from fish broth pot to thin paste slightly.

While pounding with the pestle, or with the blender or processor running at its lowest speed, drizzle in the olive oil. If the mixture seems too thick at any point, add fish broth, 1 tablespoon (15ml) at a time, until a thick, mayonnaise-like consistency is reached. Refrigerate the rouille until ready to use, up to 3 days.

Working in batches, transfer broth and all ingredients, including fish bones, to a blender. Remove top vent from blender lid, cover with a folded clean kitchen towel, and turn blender on at lowest speed. Gradually increase speed to high, then blend until as smooth as possible.

Transfer blended soup to a fine-mesh strainer set over a clean pot and, using a wooden spoon, work the broth through the strainer, scraping and pressing, until only bones and very dry fibrous material are left. Season strained broth with salt and pepper.

Return broth to heat and bring to a very gentle simmer over medium. Working in batches if necessary, season the whole and filleted fish and fully submerge in broth. Poach until just cooked through, starting with the larger whole fish that will take the longest to cook and finishing with the fillets.

If using mussels or crabs, add to gently simmering broth and cook until mussels have just opened or crabs are cooked through. Transfer to the serving platter.

Bring the platter of fish to the table along with the broth in the soup pot or a tureen. Guests can eat the soup and fish separately, or together in the same bowl. The soup is best served with baguette toasts smeared with the rouille.