Preheat Oven: Preheat your oven to 400°F (200°C).
Combine on Pan: On a large, rimmed baking sheet, add the sliced sausage, broccoli florets, chopped bell peppers, and red onion. Drizzle with olive oil.
Season: Sprinkle the Italian seasoning, salt, and pepper over the ingredients on the pan.
Toss: Use your hands or two large spoons to toss everything together until the sausage and vegetables are evenly coated with oil and seasonings.
Arrange & Roast: Spread the mixture in a single, even layer. It is important not to overcrowd the pan. Roast for 20-25 minutes, tossing once halfway through, until the vegetables are tender and caramelized at the edges.
Serve: Garnish with fresh parsley if desired and serve immediately.