Preheat your oven to 400°F (200°C) and line a large rimmed sheet pan with unbleached parchment paper for easy cleanup.
Cut the chicken breast into 1-inch bite-sized cubes, and chop the broccoli, bell pepper, and red onion into similar sized pieces to ensure even cooking.
In a small glass bowl, whisk together the extra virgin olive oil, fresh lemon juice, dried oregano, garlic powder, sea salt, and black pepper.
Place the chicken and all the chopped vegetables into a large mixing bowl and pour the lemon-herb dressing over the top.
Toss everything thoroughly with your hands or a spatula until every piece is well-coated in the oil and spices.
Spread the mixture out in a single layer on the prepared sheet pan, making sure not to overcrowd the pan so the vegetables roast rather than steam.
Bake for 18 to 22 minutes, or until the chicken reaches an internal temperature of 165°F and the vegetables are tender with slightly charred edges.