1. Preheat your oven to 400 F (200 C). Line a large baking sheet with parchment paper for easy cleanup.

2. Cut the boneless, skinless chicken breasts into uniform 1-inch cubes. Cut the broccoli into florets, bell peppers into 1-inch pieces, and the red onion into wedges. Try to make all the pieces roughly the same size so they cook at a similar rate.

3. In a large mixing bowl, combine the cubed chicken, broccoli florets, bell pepper pieces, and red onion wedges.

4. Drizzle the olive oil over the chicken and vegetables in the bowl. Sprinkle the Italian seasoning, garlic powder, salt, and black pepper over the mixture. Use your hands or a large spoon to toss everything together until all the chicken and vegetables are well coated with the oil and seasonings.

5. Transfer the seasoned chicken and vegetables to the prepared baking sheet. Spread them out in a single layer, ensuring they are not overcrowded. If necessary, use two baking sheets.

6. Place the baking sheet in the preheated oven and roast for 20-25 minutes. About halfway through the cooking time (around 10-12 minutes), give the ingredients a gentle stir or flip them with a spatula to ensure even browning. The chicken should be cooked through (internal temperature of 165 F or 74 C), and the vegetables should be tender-crisp with some slight charring.

7. Once cooked, remove the sheet pan from the oven. If desired, sprinkle with fresh chopped parsley for a burst of color and freshness. Serve hot and enjoy your easy, delicious dinner!