In a large bowl, combine the broccoli florets with olive oil, minced garlic, salt, black pepper, and red pepper flakes if using. Toss until the broccoli is well coated.
Spread the broccoli mixture evenly on a large baking sheet lined with parchment paper.
Roast in the preheated oven for 20-25 minutes, tossing halfway through, until the broccoli is tender and slightly crispy on the edges.
Once out of the oven, immediately sprinkle the grated Parmesan cheese over the hot broccoli for it to melt slightly.
Add the lemon zest and lemon juice, tossing gently to combine.