Step 1 Cut the firm-fleshed fish, notably the monkfish, into uniform pieces, removing heads, bones, skin and scraps. Cut the other cleaned and gutted whole fish crosswise into 3-inch-wide pieces.
Step 2 Place all the fish pieces and fillets in a very large mixing bowl. Add the olive oil, fennel or pastis, and 2 pinches of the crumbled saffron, season with salt and pepper, and mix well without breaking up the fish. Cover with plastic wrap and marinate in the refrigerator for 2 to 4 hours.
Step 3Heat the fish stock in a large stockpot over high heat. Add the remaining saffron and bring to a rapid boil.
Step 4Heat the oven to 250 degrees. Place the bread on a rack over a baking sheet and bake until crisp but not browned, about 15 minutes, turning over halfway through. Set aside.
Step 5Add the potatoes and fish to the pot, bring the stock back to a boil, and cook for 10 to 15 minutes or until the potatoes are tender and the fish flakes easily with a fork. Correct the seasoning, adding some cayenne, if desired. Remove 2 to 3 slices of potato and one-half cup of the stock to make the rouille.
Step 6To serve, rub the bread slices with the halved garlic cloves, arrange on a serving dish, and place on the table alongside the rouille. Remove all the fish and potatoes from the stock with a slotted spoon and arrange on a serving platter with, if desired, the cooked mussels. Discard any remaining fish bones or skin.
Step 7When ready to serve, ladle some hot stock over the fish, mussels and potatoes. Pour the remaining stock into a soup tureen. Bring the serving platter and the tureen to the table. Have the diners dab the garlic toasts (croutons) with rouille and place them on the bottom of individual shallow soup bowls. Ladle stock over the toasts and pass the platter of fish and potatoes. Sprinkle each serving with fresh thyme leaves to taste.