This rich bouillabaisse proves that pumpkins aren't just for pies. Sugar Pie and Baby Bear pumpkins are cooking varieties with infinitely more flavor and better texture than carving pumpkins, which are not recommended. Ask a produce team member about the best cooking variety available in your Whole Foods Market. Butternut squash can be substituted for pumpkin if you wish.
Using a large knife, cut off the top stem end of the pumpkin, creating a flat surface. Place the flat side down on the cutting board to stabilize the pumpkin. Cut the pumpkin in half lengthwise, then into quarters. Peel, seed, and cube the pumpkin. Set aside.
Heat the olive oil in a heavy 5- to 6-quart pan over medium-high heat. Add the fennel and leeks and sauté until tender but not brown, stirring often, about 10 minutes. Add the garlic and sauté 2 minutes more. Add the wine and cook until most of the liquid has evaporated, about 2 minutes. Add the pumpkin, chicken broth, turmeric, bay leaf, salt, and pepper; increase the heat to high and bring to a boil. Reduce the heat, cover, and simmer until the pumpkin is just tender, about 15 to 20 minutes.
Add the tomatoes and bring to a simmer. Add the halibut, scallops, and shrimp and gently stir to combine. Cover and simmer until the seafood is just opaque in the center, 4 to 5 minutes. Stir in the parsley. Ladle into bowls, garnish with a lemon wedge, and serve immediately.