Preheat your oven to 400°F (200°C) and line a large rimmed sheet pan with parchment paper for easy cleanup.
Cut the chicken breast into uniform 1-inch bite-sized pieces to ensure they cook evenly with the vegetables.
Chop the broccoli into small florets, the red bell pepper into 1-inch squares, and the red onion into thick wedges.
Place the chicken and all the chopped vegetables onto the prepared sheet pan.
Drizzle the extra virgin olive oil and lemon juice over the chicken and vegetables.
Evenly sprinkle the garlic powder, dried oregano, sea salt, and black pepper over the ingredients.
Toss everything together with your hands or tongs until every piece is well-coated in oil and seasoning.
Spread the mixture into a single layer across the pan, making sure not to crowd the pieces so they roast rather than steam.
Roast in the center of the oven for 20 to 25 minutes, or until the chicken reaches an internal temperature of 165°F and the vegetables are tender with charred edges.