1/2 cup olive oil
4 leeks, white and pale green parts only, chopped
2 medium-sized yellow or green bell peppers, seeded and chopped
2 celery ribs, thinly sliced
3 garlic cloves, finely chopped
1 tablespoon fennel seed
1 to 2 teaspoons dried red pepper flakes
2 teaspoons ground turmeric
1 1/2 teaspoons dried thyme
5 cups water
2 cups dry white wine
1 can (28-ounce) diced tomatoes with juice
1/3 cup chopped fresh flat-leaf parsley, plus more for sprinkling
24 clams, scrubbed
24 mussels, scrubbed (debearded if necessary)
2 1/2 pounds lean white fish, cut into large chunks (haddock, cod, pollock, or a combination) salt pepper
Heat the oil in a very large soup pot. Add the leeks, bell peppers, celery, and garlic and cook over medium heat until the vegetables soften, about 10 minutes.
Add the fennel seed, red pepper flakes, turmeric, and thyme and cook, stirring, for 2 minutes.
Add the water, wine, tomatoes, and parsley. Bring to a boil, reduce the heat to medium-low, and simmer, uncovered, for about 15 minutes to blend flavors. The base can be made up to a day ahead and refrigerated. Reheat before proceeding.)
Add the clams and mussels to the stew base and cook gently until the shells begin to open, 5 to 8 minutes. Add the fish and simmer until it is cooked through and the mollusk shells are completely open, about 5 minutes. Season with the salt and pepper to taste. Set aside in the refrigerator for at least 2 hours.
Reheat if necessary. Ladle into bowls, sprinkle with the parsley, and serve.