Preheat the oven to 400°F. Lightly grease a 9x13-inch baking dish.

Bring a large pot of salted water to a boil. Add the halved Brussels sprouts and cook for about 5 minutes, until slightly tender but still firm.

Drain the Brussels sprouts thoroughly and set aside.

Melt the butter in a large saucepan over medium heat.

Add the minced garlic and cook for about 30 seconds, or until fragrant.

Sprinkle in the flour and whisk constantly for 1 minute to form a roux.

Slowly pour in the heavy cream while whisking continuously.

Cook for 2–3 minutes, stirring frequently, until the sauce begins to thicken.

Stir in the nutmeg, salt, and black pepper.

Remove the saucepan from the heat and stir in 1½ cups Gruyère cheese until melted and smooth.

Add the Brussels sprouts to a large bowl. Pour the cheese sauce over the sprouts and gently toss until evenly coated.

Transfer the mixture to the prepared baking dish.

Sprinkle the remaining ½ cup Gruyère and the Parmesan evenly over the top.

Bake for 20–25 minutes, or until the casserole is hot and bubbly and the cheese is golden.

Let rest for about 5 minutes before serving.