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 simmer over medium heat. Add cleaned mussels, cover, and steam for 4–6 minutes until opened. Strain mussels, reserving broth, and set aside.

In a saucepan, melt 4 tablespoons of unsalted butter over low heat. Once bubbling, add 1 minced shallot and cook until translucent, about 2 minutes. Gradually whisk in 1/4 cup of flour, cooking for 1–2 minutes until golden.

Slowly whisk in reserved mussel broth and 1 cup of heavy cream into roux, ensuring no lumps. Raise heat and simmer for 5–6 minutes until thickened. Add salt, pepper, and 1 tablespoon of Dijon mustard, adjusting seasoning.

Reduce heat to low and gently poach shrimp, scallops, and white fish in sauce for 3–4 minutes until opaque. Fold in reserved mussels and heat through. Stir in chopped parsley and thyme.

Preheat oven to 400°F (200°C). Spoon seafood mixture into ramekins or baking dish, compacting slightly.

In a bowl, combine 1 cup of breadcrumbs and 1/2 cup of grated Gruyère cheese. Drizzle with 2 tablespoons of melted butter and mix thoroughly. Sprinkle over cassolette, covering completely.

Bake in the preheated oven for 12–15 minutes, until golden and bubbling. Switch to broil for the last 2-3 minutes for added crispness.