In a large bowl, toss the halved Brussels sprouts with olive oil, salt, and pepper until evenly coated.
Spread the Brussels sprouts onto a baking sheet in a single layer.
Roast in the preheated oven for 20-25 minutes, or until the sprouts are golden brown and crisp-tender, tossing halfway through.
While the sprouts are roasting, melt the butter in a saucepan over medium heat.
Add the minced garlic and red pepper flakes to the melted butter, cooking for 1-2 minutes until fragrant. Be careful not to burn the garlic.
Once the Brussels sprouts are done, remove them from the oven and drizzle the garlic butter mixture over them, adding the lemon juice for a fresh kick.
Sprinkle grated Parmesan cheese generously over the Brussels sprouts and return them to the oven for an additional 5 minutes, allowing the cheese to melt slightly.
Remove from the oven, garnish with chopped parsley, and serve warm.
Notes
For extra flavor, add more red pepper flakes if you like it spicy.