In a large pot, combine a cup of dry white wine, minced garlic, and fresh thyme. Bring to simmer over medium heat. Add scrubbed mussels, cover the pot, and steam for 5-7 minutes. Discard any unopened mussels and reserve cooking liquid.

In a medium saucepan, melt 4 tablespoons of unsalted butter over low heat. Add finely minced shallots and sauté until translucent (3-4 minutes). Sprinkle in flour, stirring continuously for another 2 minutes to create a roux.

Gradually whisk in reserved mussel cooking liquid and 1 cup of heavy cream, stirring. Turn heat to medium and simmer for 5-7 minutes until thickened and bubbly.

Gently add shrimp, scallops, and white fish into the sauce. Cook for 3-4 minutes until seafood is just opaque.

Fold in steamed mussels and chopped parsley. Spoon mixture into ramekins or a baking dish.

In a small bowl, combine breadcrumbs, grated Gruyère cheese, and melted butter. Coat well.

Preheat oven to 400°F (200°C). Sprinkle breadcrumb mixture over seafood filling. Bake for 12-15 minutes until golden and bubbly.

Remove from oven and let sit for a couple of minutes before serving. Best enjoyed warm.