Heat oven to 400 degrees; brush bread liberally with olive oil, and bake on a sheet, turning once, until golden and crisp, about 5 minutes. Set aside.

Add enough olive oil to a Dutch oven, deep skillet or shallow pot to make a thick layer (don't skimp) on the bottom. In it, cook onion, garlic, celery, carrot, potato, fennel and saffron until glossy. Add stock and tomato and bring to a moderate boil; cook until thick and stewy rather than soupy. Season to taste; it should be so delicious that you don’t even care whether you add fish.

Lower heat to a simmer, and, as you add fish, adjust heat so that the liquid continues to bubble gently. Add fish in order of how long they will take to cook. Monkfish, striped bass and squid are fish that might require more than a few minutes, so add them first. About five minutes later add clams and mussels, holding back any fish that has been cooked or will cook in a flash. When mollusks open, add remaining fish. Cut scallops into quarters and place in the bottom of 4 bowls.

Add pastis if you're using it; taste and adjust seasoning. Ladle hot soup and fish over the scallops, distributing clams and mussels evenly. Garnish and serve with croutons and rouille, if you're using.