Preheat oven to 400F. Line a baking sheet with aluminum foil for easier cleanup, and add the sausage, bell peppers, zucchini, tomatoes, and evenly drizzle with olive oil.
Evenly sprinkle with onion flakes, smoked paprika, parsley, oregano, salt, pepper, and toss with your hands to coat and distribute seasonings. Scatter vegetables in a flat layer on the pan.
Bake for about 40 minutes, or until vegetables are tender; stir halfway through baking to ensure even cooking. Start checking at 30 minutes for doneness.
Sprinkle with Parmesan (if desired) and serve immediately.