What goes into a traditional bouillabaisse? That depends on whom you ask. But a pot typically includes at least four kinds of fish -- some firm and some soft -- as well as fennel, garlic, olive oil, onion, parsley, saffron, and tomatoes.
Marinate fish: Stir together saffron and pastis; set aside. Stir together sliced fennel, 1/4 cup fronds, and 2 tablespoons garlic. Divide fennel mixture between two 9-by-13-inch nonreactive baking dishes. Cut fish into uniform portions, each 4 to 5 inches long and 2 to 3 inches wide. Generously season with salt. Put thick pieces in one baking dish and thin ones in the other. (They will be poached in separate batches later.) Pour 2 tablespoons pastis mixture and 1/4 cup oil into each dish; turn fish to coat. Cover; marinate in refrigerator 2 hours, turning once.
Meanwhile, prepare stock: Cover fish heads and bones with cold water; let soak. Heat remaining 1/4 cups oil in a tall, 10-quart stockpot (that includes a steamer basket for later use) over medium heat until hot but not smoking. Add onion, leeks, celery, chopped fennel, 1 tablespoon salt, and remaining garlic. Cook, stirring, until translucent, about 7 minutes.
Add tomato paste; stir well. Stir in 2 cups wine, remaining pastis mixture, orange zest, and juice. Tie together bay leaves, parsley, thyme, and rosemary sprigs with kitchen string; add to pot. Stir broth. Bring to a boil.
Drain and rinse fish heads and bones; add to pot. Pour in 10 cups cold water (liquid should come to within 3 inches of rim of pot). Bring to a boil; let boil for 10 minutes. Skim foam from surface.
Reduce heat to medium-high. Stir in tomatoes. Cook until stock is slightly reduced, about 20 minutes. Raise heat to high; boil until olive oil binds with liquids, 20 to 30 minutes. Meanwhile, bring marinated fish to room temperature.
Steam shellfish: Put mussels and shrimp in a large, shallow pot (steaming the shrimp in their shells enhances their flavor). Add 1/2 cup wine and 1/4 cup fennel fronds; cover. Cook over high heat until shrimp are pink and cooked through and mussels are open (check pot frequently after 4 minutes). Transfer cooked shellfish with a slotted spoon to a large bowl (after 7 minutes, discard any mussels that remain closed); reserve pot. Cover bowl with foil.
Pass stock through a food mill: Using tongs or a slotted spoon, lift out heads and large bones; discard. Discard herbs. In batches, pass stock through a food mill set over the pot of shellfish juices, extracting as much liquid as possible. (If you don't have a food mill, pass stock through a fine sieve, firmly pressing on solids with the back of a ladle.) Discard solids.
Pass broth through a chinois or a fine sieve into stockpot. Press on solids; discard. Reserve 1/2 cup broth for rouille.
Cook potatoes: Bring broth to a boil. Place potatoes in steamer basket; submerge in broth. Reduce heat to medium; cook until tender, about 8 minutes. Transfer to a serving platter; cover with foil.
Cook fish: Return broth to a boil. Place thick fish pieces in basket; submerge in broth. Reduce heat to low. Poach until cooked through, about 6 minutes. Transfer to platter; cover with foil. Return broth to a boil; place thin fish pieces in basket; submerge in broth. Reduce heat to low; poach until cooked through, 4 to 5 minutes. Transfer to platter; pour any juices from platter and shellfish bowl into broth. Cover fish and shellfish with foil.
Pass broth through a chinois or fine sieve into a serving bowl. Press on solids with a ladle; discard solids.
Serve: Ladle about 3/4 cup broth into each bowl. Serve with rouille-topped croutons. Follow with fish and potatoes, served with any remaining broth.