Preheat your oven to 425°F (220°C). Wash and trim the Brussels sprouts, cutting them in half.

In a large mixing bowl, toss the Brussels sprouts with olive oil, salt, and black pepper until well-coated.

Spread the Brussels sprouts on a baking sheet in a single layer and roast for about 20 minutes, shaking halfway through.

While the sprouts roast, melt the butter in a medium saucepan over medium heat. Add minced garlic and sauté for 1-2 minutes until fragrant.

Sprinkle in the flour, whisking continuously for about 2 minutes. Gradually add the milk, whisking until smooth.

Once thickened, stir in the cheddar cheese until melted and creamy. Season with salt and pepper to taste.

Pour the cheese sauce over the roasted Brussels sprouts and mix gently. Transfer to a greased baking dish.

Sprinkle Parmesan cheese over the top and bake for an additional 10-12 minutes until bubbly and golden brown.

Let cool slightly, then garnish with parsley and red pepper flakes before serving.