Trim off and discard stalks, root ends, and bruised areas from fennel. Chop and reserve green tops. Rinse heads; cut in half lengthwise, then slice.
Heat oil in a 5- to 6-quart pan over medium-high heat; add fennel, onion, and garlic and stir often until soft, 10 to 12 minutes. Add wine; cook until most of liquid has evaporated, about 2 minutes. Add broth, clam juice, Pernod (if using), pumpkin, saffron, bay leaf, salt, and pepper; increase heat to high and bring to a boil. Reduce heat, cover, and simmer just until pumpkin is tender when pierced, 15 to 20 minutes.
3. Add tomatoes; bring to a simmer. Gently stir in fish; simmer until opaque in center (cut to test), 4 to 5 minutes. Stir in parsley. Ladle into bowls and top with 1 tablespoon rouille and fennel tops.
Rouille: In a bowl, stir together 1/2 cup mayonnaise, 1 tablespoon tomato paste, 2 teaspoons lemon juice, 1 minced garlic clove, 1/4 teaspoon cayenne, and salt and pepper to taste. Makes about 1/2 cup.