Heat the olive oil in a Dutch oven set over medium heat until hot. Add the fennel, onion, and a pinch of salt, sweating until softened, 6-8 minutes.
Add the garlic, fennel seeds, and crushed red pepper; cook and stir until fragrant, about 1 minute.
Stir in the tomatoes (and their juice), stock, wine, and some salt and pepper to taste. Bring to a boil and then reduce to a simmer; cook, stirring occasionally, for 30 minutes.
Add the clams, mussels, halibut, and shrimp to the soup; return to a simmer, cover, and cook until the clams and mussels have opened and the shrimp and fish are cooked through, about 5 minutes; discard any mussels that don't open.
Stir in the parsley and some salt and pepper to taste. Serve the cioppino straight from the Dutch oven.
