In a large pot, combine 1 cup of dry white wine, 2 minced garlic cloves, and a few sprigs of fresh thyme. Bring to a gentle simmer over medium heat, then add 1 pound of scrubbed and debearded mussels. Cover the pot and steam until mussels open, about 4-6 minutes.

In a medium saucepan, melt 4 tablespoons of unsalted butter over medium heat, add 1 finely chopped shallot, and sauté until translucent, about 2-3 minutes. Stir in 1/4 cup of all-purpose flour, cooking for 1-2 minutes to form a roux.

Gradually whisk in 1 cup of white wine, followed by 1 cup of seafood stock and 1 cup of heavy cream. Bring the mixture to a gentle simmer, stirring frequently until it thickens, about 5-6 minutes.

Reduce heat to low and carefully add 1 pound of shrimp, 1 pound of scallops, and 8 ounces of flaked white fish to the sauce. Gently simmer for 3-4 minutes, then fold in the steamed mussels and chopped fresh parsley.

Preheat your oven to 400°F (200°C). In individual ramekins or a large baking dish, divide the seafood mixture evenly. Spoon enough of the creamy sauce over the seafood to coat them generously.

In a bowl, mix 1 cup of breadcrumbs with 1 cup of grated Gruyère cheese and 2 tablespoons of melted butter. Sprinkle this mixture generously over the assembled seafood.

Place the cassolette in the preheated oven and bake for 12-15 minutes until the top is golden brown and bubbling. Broil for an additional 1-2 minutes if desired for a crispier finish.