1. Cover potatoes with water in a medium saucepan. Bring to a boil. Reduce to a simmer. Cook until tender, about 15 minutes. Drain; let cool completely.

2. Preheat oven to 475 degrees. Heat sherry in a small saucepan over medium-low heat until warm, 2 to 5 minutes. Add saffron, and stir until it has dissolved. Set aside.

3. Heat oil in a cast-iron skillet in oven 5 minutes. Season fish with salt and pepper; set aside. Remove skillet from oven, and add fish, skin side down. Drizzle with the sherry mixture. Add sausage and shallots. Roast 10 minutes. Remove from oven; brush fish with pan juices. Add clams and mussels. Roast until fish is cooked through and clams and mussels have opened, about 10 minutes (discard any shellfish that remain closed). Transfer skillet to a wire rack. Drizzle with lemon juice. Let cool 15 minutes.

4. Arrange tomato slices on a platter. Cut cooled potatoes into 1/4-inch-thick rounds; arrange over tomatoes. Top with seafood mixture; reserve 2 tablespoons pan juices.

5. Make the dressing: Whisk reserved pan juices, lemon juice, parsley, and basil in a medium bowl. Gradually add oil, whisking constantly until emulsified.

6. Drizzle salad with dressing. Serve with saffron aioli and toasted baguette.