Preheat oven to 400°F (200°C) and line a large sheet pan with parchment paper.
Cut chicken breast into bite-sized pieces and chop the broccoli, bell pepper, and red onion into uniform chunks.
In a large bowl, whisk together the olive oil, juice of half the lemon, oregano, garlic powder, sea salt, and black pepper.
Add the chicken and vegetables to the bowl and toss thoroughly until everything is evenly coated in the marinade.
Spread the mixture in a single layer on the prepared sheet pan, ensuring no pieces are overlapping to allow for even roasting.
Slice the remaining half of the lemon into thin rounds and tuck them between the chicken and vegetables on the pan.
Roast for 20-25 minutes, or until the chicken is cooked through and the vegetables are tender and slightly charred.