Heat oil in a large, wide pot over medium-high heat. Sauté leek and onion until translucent, 2 minutes. Add garlic, then fennel slices from under fish on platter. Sauté 2 minutes, then add tomatoes, stock, wine, bay leaf, thyme, pepper, and salt.
Remove fish from platter; set aside. Lift off fennel stalks and fronds and discard. Scrape marinade into broth. Bring to a boil, covered; then simmer, covered, until fennel slices are meltingly soft, 30 minutes. Meanwhile, bring 1 qt. water to a boil.
Bring broth to a rolling boil. Lay in the halibut and add enough boiling water to just cover fish. Cook until just opaque, 5 to 7 minutes; transfer to a platter and cover.
Add thinner fish fillets and spot prawns and cook just until opaque, 2 minutes; transfer to platter as done. Add crab and mussels and cook just until mussels open, 5 minutes. Transfer both to platter. Add squid and cook just until opaque, about 1 minute; transfer to platter. Bathe platter with a ladle of broth. Remove bay leaf and thyme.
Ladle about 1 cup broth each into big soup bowls (keep broth hot for a second serving, covered). Bring bowls, platter, rouille, and toasts to the table. Put a little of each seafood in every bowl and top with a dollop of rouille. Serves 8 to
Make ahead: Add fish to remaining broth and keep, chilled, up to 5 days.
When choosing your seafood, try to vary the textures and flavors. Some should be firm, others soft; some mild, others briny. Find them at a good seafood shop or your farmers' market.
Alaskan snow crab legs: Very firm, lobsterlike texture; sweet and mild. Usually only available cooked.
California mussels: Briny flavor; soft, melting texture with a bit of chew.
California squid (calamari): Firm but tender; sweet yet meaty. It's neither fish nor shellfish, really, but it gives great texture and flavor to the soup.