Prepare the Brussels Sprouts

1. Preheat the Oven: Preheat your oven to 375°F (190°C).
2. Roast the Sprouts: Toss the Brussels sprouts with olive oil, salt, and pepper. Spread them on a baking sheet and roast for 15 minutes until lightly browned.
3. Prepare the Sauce: In a small saucepan, combine the cream, Gruyère cheese, Parmesan cheese, garlic, mustard, nutmeg, salt, and pepper. Stir over medium heat until the cheese is melted and the sauce is smooth.

Assemble the Gratin

1. Layer the Dish: Transfer the roasted Brussels sprouts to a greased baking dish. Pour the cheese sauce evenly over the sprouts.
2. Add the Topping: In a small bowl, mix the breadcrumbs, melted butter, and Parmesan. Sprinkle the mixture over the Brussels sprouts and sauce.

Bake the Gratin

1. Bake: Bake the gratin in the preheated oven for 20 minutes, or until the topping is golden brown and crispy.
2. Cool: Let the gratin cool for a few minutes before serving.