Instructions:

1. Preheat Oven: Preheat the oven to 400°F (200°C).
2. Blanch the Brussels Sprouts: In a pot of salted boiling water, blanch the Brussels sprouts for 3-4 minutes. Drain and set aside.
3. Make the Cheese Sauce: In a saucepan, melt the butter over medium heat. Add the garlic and cook for 30 seconds. Stir in the flour and cook for another minute to form a roux. Gradually whisk in the milk and cook until the sauce thickens. Stir in the Gruyère and Parmesan cheeses until melted and smooth.
4. Assemble the Gratin: In a baking dish, layer the Brussels sprouts and pour the cheese sauce over them. Top with breadcrumbs and a bit more Parmesan if desired.
5. Bake: Bake for 20-25 minutes until the top is golden and bubbly.
6. Serve: Serve as a hearty side dish for any meal.

Enjoy!