For the croutons
Position a rack in the center of the oven and heat the oven to 450°F. Arrange the baguette rounds on a large rimmed baking sheet and bake until crisp and lightly golden, about 10 minutes. Turn off the oven, open the door partially, and let the croutons cool and finish drying in the oven. Store in an airtight container for up to 1 day.
For the fish broth
Put the mussels and 1 cup of water in a 4- to 5-quart pot. Cover, bring to a boil over high heat, and cook until all the mussels have opened, about 2 minutes. Set a strainer over a small bowl. Shuck the mussels into the strainer, discarding the shells. Press on the mussels to squeeze out any remaining liquid;discard the solids. Strain the mussel-cooking liquid into the bowl and set aside.
Heat the oil in a wide 8- to 10-quart pot over medium heat. Add the garlic and onions, cover, and cook, stirring occasionally, until softened but not colored, about 8 minutes. Add the fennel, leeks, and celery and cook, covered, stirring occasionally, until completely soft but not colored, about 20 minutes.
While the vegetables cook, use a chef’s knife or kitchen scissors to cut the fish skeletons into 4- to 5-inch pieces. Add the fish skeletons, heads, and trimmings to the pot, along with the tomatoes, parsley, thyme, bay leaves, orange zest, pastis, fennel seeds, peppercorns, 2 tsp. salt, and the reserved mussel liquid. Add enough cold water to just cover the solids when you press them down gently (about 1 quart). Bring to a gentle simmer over medium heat and cook, uncovered, until you can easily break the fish parts with a wooden spoon, about 30 minutes. Break apart as many of the fish parts as you can with the spoon.
Put a food mill over a large bowl and, working in batches, grind the cooked broth ingredients, discarding the solids left in the food mill between batches. Clean the pot and put a fine strainer over it. Working in batches, strain the broth, pushing hard on the solids with a ladle to squeeze out all the liquid, and discarding the solids between batches.
Crumble the saffron into the broth and bring to a boil to infuse the saffron. Turn off the heat and use an immersion blender to emulsify the broth, or whisk vigorously. Season to taste with salt. Set aside. A thin, greasy skin will form on top of the broth as it sits—skim it off before cooking the potatoes.
For the fish
While the broth cooks, cut the fish fillets so you have 8 pieces of each type of fish. Use a scale; each piece should be about 1-3/4 oz. (If you have more than 14 oz. of any type of fish, trim off the thinner parts—the tail pieces and belly flaps—first and add them to the fish broth.) Put the fish in a large baking dish, keeping the same types of fish together (so you can keep track of each type).
Marinate the fish 45 minutes before you plan to finish the stew: Drizzle the fish with the pastis and sprinkle with 1 tsp. salt and 1/2 tsp. pepper. Turn the fish to coat evenly, still keeping the same types of fish together. Let the fish marinate in the refrigerator for 30 minutes; then let sit at room temperature for 15 minutes before cooking.
For the potatoes
Put the potatoes in a 4- to 5-quart pot. Add enough of the fish broth to cover completely, about 3 cups. Bring to a boil over high heat, reduce the heat, and simmer until slightly undercooked and still a little firm in the centers, about 8 minutes. Set aside off the heat—the potatoes will finish cooking in the hot broth.
For the rouille
In a mortar, mash the garlic and 1/4 tsp. each salt and cayenne with a pestle. Mash in the breadcrumbs and broth. Mash in the potato. Mash in the oil a little at a time. The rouille should be fairly thick and spreadable. Don’t overmash, or it will get gummy. Season to taste with more salt and cayenne; it should be pleasantly spicy. (Alternatively, mash the garlic and salt to a paste on a cutting board with the side of a chef’s knife. Transfer the paste to a bowl and use a fork to mash in the remaining ingredients. Don’t use a food processor, because it makes a gummy rouille.)
To finish the stew
Warm 8 wide, shallow soup bowls and a sauce pitcher in a low oven. Top 16 of the croutons with a thin layer of the rouille (you won’t use it all). Transfer the remaining rouille and croutons to 2 bowls to pass at the table.
Reheat the potatoes in their broth, but do not boil, or they will break apart. Meanwhile, bring the large pot of fish broth to a brisk simmer over medium-high heat. Use a slotted spatula to divide the potatoes among the warm serving bowls. Add the broth remaining from the potatoes to the large pot of fish broth and return to a brisk simmer.
Add the thicker pieces of fish to the simmering broth and then add the thinner pieces. If possible, try to keep the same types of fish together for easier plating. Poach until the thinner pieces are just cooked through, about 6 minutes.While the fish poaches, the broth should stay at a bare simmer; adjust the heat as necessary. Working quickly, use the slotted spatula to distribute the thinner pieces of fish, skin side up, among the bowls. Check to make sure the thicker pieces are cooked through and distribute them among the bowls. Each serving should get 4 types of fish.
Return the broth to a boil over high heat. Remove from the heat and use an immersion blender or whisk to emulsify the broth. Season to taste with salt and pepper. Ladle a generous amount of broth over each serving of fish. Transfer any remaining broth to the warm pitcher.
Nestle 2 rouille-topped croutons in each serving bowl and sprinkle with parsley. Serve immediately, passing the remaining croutons, rouille, and broth at the table.
Make Ahead Tips
You can toast the croutons a day ahead, but the other components are best eaten the day they’re made.
For the full Marseilles experience, serve with a dry rosé from Provence and a tangy lemon tart for dessert.
nutrition information (per serving):
Photo: Scott Phillips