Add a little French flair to your table with this shrimp, scallop, mussel, and whitefish-loaded soup. Check fresh mussels carefully before adding to the broth and discard any mussels that gap open and do not close when tapped lightly with a finger.
Thaw shrimp and scallops, if frozen; cut up any large scallops. In a large saucepan cook potatoes in a large amount of boiling water for 10 to 15 minutes or just until potatoes are tender when pierced with a fork. Drain; set aside. Scrub mussels under cold running water. Using your fingers, pull out beards visible between mussel shells; discard. In a large bowl combine 2 cups water and 3 tablespoons salt; soak mussels for 15 minutes. Drain and rinse thoroughly; drain. Repeat two more times with fresh water and salt. Drain and rinse mussels thoroughly.
Cut off and discard stalks from fennel bulb half. If desired, reserve wispy fronds. Finely chop fronds; set aside. Remove and discard any wilted outer layers; cut a thin slice from base of bulb half; discard. Cut bulb half in half lengthwise. Cut core out of each piece; discard. Finely chop fennel bulb.
In a 5-quart Dutch oven cook and stir fennel, onion, and garlic in hot oil until tender. Carefully stir broth, tomatoes, the 1/2 teaspoon salt, the lemon peel, saffron threads, and cayenne pepper into Dutch oven. Bring to boiling; add potatoes, shrimp, scallops, and fish. Return to boiling; reduce heat. Simmer, covered, about 5 minutes or until mussel shells open. Discard any mussels that don't open. Ladle soup into bowls; top each serving with Garlic-Parmesan Croutons. If desired, garnish with reserved chopped fennel fronds.
Preheat oven to 350 degrees F. Line a 15x10x1-inch baking pan with parchment paper. In a large bowl combine bread, olive oil, Parmesan cheese, and ground black pepper. Toss to combine. Spread cubes into prepared pan. Bake about 20 minutes or until golden, stirring once or twice. Cool in pan on wire rack.