Preheat the oven to 400 degrees F.
Put the broccoli on a baking sheet and drizzle with olive oil. Toss with the crushed red pepper flakes and lemon zest. Season with salt and pepper to taste.
Roast until the broccoli begins to brown and crisp in places, 10 to 12 minutes. Finish by squeezing the juice from one of the lemon halves over the broccoli.