How to Make It
Combine first 4 ingredients in a bowl; stir well. Cover and chill.
Heat the olive oil in a large Dutch oven over medium-high heat. Add sliced fennel, onion, thyme, and 2 garlic cloves, and sauté until tender.
Add water and the next 6 ingredients (water through tomato purée) to fennel mixture, and bring to a boil. Cook for 15 minutes or until potatoes are crisp-tender. Add the halibut, scallops, and shrimp; cover, reduce heat, and simmer for 5 minutes or until seafood is done.
Ladle soup into individual bowls, and garnish with parsley, if desired. Serve with toast topped with tomato-mayonnaise mixture.