Preheat the oven to 220°C (425°F). Position the rack in the upper third of the oven.
Cut the broccoli into even florets and pat them completely dry with a clean kitchen towel.
Toss the florets in a large bowl with the olive oil, salt, black pepper, and red pepper flakes until evenly coated.
Spread the florets on a large sheet pan in a single layer with clear gaps between each piece. Use two pans if needed.
Roast for 18 to 20 minutes without opening the oven door.
Scatter the sliced garlic across the pan. Return to the oven for 5 to 7 more minutes until the garlic is golden and the broccoli stems are tender when pierced with a knife.
Remove from the oven. Squeeze lemon juice over the hot broccoli, scatter the lemon zest, and top with parmesan if using. Serve immediately.