Preheat your oven to 375°F.

Melt the unsalted butter in a large pan over medium heat. Add the minced shallots and garlic, sautéing until they become translucent and fragrant, about 3-4 minutes.

Stir in the flour and cook for 1-2 minutes until it's lightly golden.

Add the dry white wine and seafood stock gradually, stirring continuously until a smooth mixture forms, about 2-3 minutes.

Pour in the heavy cream and bring to a gentle simmer while stirring, allowing it to thicken for about 5 minutes.

Mix in the shrimp, scallops, white fish, and mussels. Cook for about 3-4 minutes until just cooked through.

Incorporate the Dijon mustard, chopped thyme, parsley, salt, and pepper.

Transfer the seafood mixture into a baking dish, spreading it out evenly.

Combine the breadcrumbs and grated Gruyère cheese with melted butter in a bowl, then sprinkle over the seafood.

Bake in the preheated oven for about 20-25 minutes, or until the top is golden brown and bubbly.