Make the rouille: Puree the mayonnaise, roasted red peppers, olive oil, lemon juice, garlic and a pinch of salt in a blender until smooth. Transfer to a small bowl; refrigerate until ready to serve.
Prepare the bouillabaisse: Grate the tomato flesh on the large holes of a box grater down to the skins; reserve the skins separately. Peel and devein the shrimp, reserving the shells and tails for the broth; clean and debeard the mussels. Refrigerate the seafood until ready to use.
Make the broth: Heat 3 tablespoons olive oil in a large pot over medium-high heat. Add the shrimp shells and tails and cook, stirring, until bright red, about 3 minutes. Stir in the tomato paste until incorporated, about 30 seconds. Add the clam juice, 4 cups water, the reserved tomato skins, the orange zest and 1/4 cup of the sliced fennel. Bring to a boil, then reduce the heat to medium; simmer until the liquid is reduced to about 4 cups, 12 to 15 minutes. Strain the broth through a fine-mesh sieve set over a bowl, pressing on the solids to extract the liquid; reserve the broth.
Wipe out the pot and heat the remaining 3 tablespoons olive oil over medium heat. Add the remaining fennel, the shallot, garlic and a pinch of salt; partially cover and cook, stirring occasionally, until the fennel begins to soften, about 5 minutes. Add the reserved grated tomato flesh and cook, stirring, until slightly thickened, about 2 minutes. Stir in the saffron, then add the wine and simmer until the liquid is reduced by about two-thirds, about 4 minutes. Add the reserved broth and return to a simmer.
Add the mussels to the pot; cover and cook 4 minutes. Add any thick pieces of fish and cook, covered, 2 minutes. Add the remaining fish and the shrimp and cook, covered, until the mussels open and the fish and shrimp are just cooked through, 3 to 4 more minutes. (Discard any mussels that do not open.) Season with salt and pepper.
Spread the baguette with the rouille and serve with the bouillabaisse. Sprinkle with parsley.