Step 1

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.

Step 2

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.

Step 3

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.