Begin by combining a cup of dry white wine, minced garlic, and fresh thyme in a large pot over medium heat. Bring the mixture to a simmer and add your mussels, covering the pot tightly. Steam for about 5-7 minutes or until the mussels open. Reserve the cooking liquid by straining it through a fine mesh sieve to remove any impurities.

In a saucepan, melt 4 tablespoons of unsalted butter over medium heat until bubbling gently. Add finely chopped shallots and sauté for about 3-4 minutes until they become soft and translucent. Next, whisk in a quarter cup of flour to form a roux, stirring constantly for 2-3 minutes to eliminate the raw flour taste and achieve a light golden color.

Gradually incorporate the reserved mussel liquid and an additional cup of heavy cream into the roux, whisking continuously to avoid lumps. Stir in another half cup of dry white wine and bring the sauce to a gentle simmer. Cook for 8-10 minutes, allowing it to thicken until it coats the back of a spoon; this creamy sauce will be the base for your Gourmet Seafood Cassolette.

Once the sauce has thickened, reduce the heat to low and add Dijon mustard, fresh thyme, salt, and pepper to taste. Gently fold in the shrimp, scallops, and flaky white fish, simmering for about 5-6 minutes until the seafood is just opaque. Finally, stir in the steamed mussels and freshly chopped parsley, ensuring everything is well-combined before removing it from the heat.

Preheat your oven to 400°F (200°C). In individual ramekins or a baking dish, spoon the seafood mixture evenly, filling each about three-quarters full. Make sure the creamy sauce surrounds the tender seafood, creating a delightful visual.

In a small bowl, combine a half cup of breadcrumbs with a quarter cup of grated Gruyère cheese and two tablespoons of melted butter. Sprinkle this mixture generously over the seafood in each ramekin. Bake the cassolette in the preheated oven for 12-15 minutes or until the tops are golden brown and bubbling.