Preheat the oven to 350°F. Line a baking sheet with parchment paper.
Place the broccoli florets in a large bowl and drizzle olive oil evenly over them. Add the salt and pepper to taste. Gently toss the broccoli until the oil, salt, and pepper are evenly distributed.
Spread the broccoli florets out in a single layer on the prepared baking sheet. Place one half of the lemon on the tray. Bake for 45 minutes.
Take the baking sheet out of the oven. Sprinkle chili flakes and Parmesan cheese over the broccoli. Squeeze the unbaked lemon half onto the broccoli. Return the sheet to the oven to melt the cheese, about an additional 5 minutes.