Creating Gourmet Seafood Cassolette is a straightforward process when you follow these clear steps:

1. Preheat your oven to 375°F (190°C). Prepare your cassolette dishes or an appropriate baking dish by buttering them lightly.
2. Sauté Aromatics: In a large skillet, melt butter over medium heat. Add the finely chopped onion and sauté until translucent (about 5 minutes).
3. Add Garlic: Stir in the minced garlic and continue cooking for another 1-2 minutes until fragrant.
4. Prepare the Sauce: Sprinkle the flour into the skillet, stirring to create a roux. Gradually add the seafood stock and heavy cream, whisking constantly until the mixture thickens (about 5 minutes).
5. Season: Mix in the Dijon mustard, Old Bay seasoning, salt, and pepper, adjusting according to your taste preferences.
6. Incorporate Seafood: Add the shrimp and scallops to the creamy sauce. Stir gently to coat the seafood evenly.
7. Transfer to Dishes: Divide the seafood mixture evenly between your cassolette dishes.
8. Cheese It Up: Sprinkle the grated Gruyère cheese generously over the top of each cassolette. If desired, add breadcrumbs for extra crunch.
9. Bake: Place in the preheated oven and bake for 20-25 minutes or until the cheese is bubbly and golden brown.
10. Cool Slightly: Once done, remove from the oven and let it cool for about 10-15 minutes.
11. Garnish and Serve: Finally, sprinkle the chopped parsley over the cassolette for a fresh touch.

With these easy-to-follow instructions, you’ll create an incredible Gourmet Seafood Cassolette that will amaze and delight anyone at your table.