Preheat oven to 400º. In a small bowl, combine olive oil, garlic, red pepper flakes and parsley and season with salt and pepper. Toss in shrimp and set aside to marinate.
Meanwhile, on a rimmed half sheet pan, toss onions, broccoli, and red bell pepper with a drizzle of olive oil and season with salt and pepper. Spread out in a single layer and roast for 20 minutes.
Pour shrimp and marinade over roasted vegetables and toss together. Spread out in a single layer and roast again until shrimp is pink and lightly toasted, 5 minutes. Serve immediately.
