1. Preheat a cast-iron griddle or large skillet over high heat. Place broccoli on dry griddle and char until blistered on 1 side. Turn and char other side. Transfer to a large heatproof bowl.
2. In a separate large skillet, warm oil over medium heat. Add garlic and red pepper flakes and cook, stirring frequently, until garlic is golden brown, about 2 minutes. Pour oil mixture over broccoli and turn to coat. Let marinate for 10 minutes. Drizzle with lemon juice and season with salt and lemon zest. Serve hot or at room temperature.