Preheat your oven to 400°F (200°C) and line a large sheet pan with parchment paper for easy cleanup.
Cut the chicken breast into bite-sized 1-inch cubes and chop the broccoli, bell pepper, and red onion into similar sized pieces.
In a small bowl, whisk together the olive oil, lemon juice, dried oregano, garlic powder, sea salt, and black pepper until well combined.
Place the chicken and vegetables onto the prepared sheet pan and pour the lemon-herb dressing over the top.
Toss everything thoroughly with your hands or tongs to ensure every piece is evenly coated in the oil and spices.
Spread the mixture into a single layer, ensuring the chicken pieces are not overlapping so they brown properly.
Roast in the center of the oven for 20 to 25 minutes, or until the chicken is cooked through and the vegetables are tender-crisp with slightly charred edges.
Remove from the oven and let rest for 2 minutes before serving to allow the juices to redistribute.