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 1-inch bite-sized pieces and place them in a large mixing bowl.
Chop the broccoli into small florets, slice the red bell pepper into thin strips, and cut the red onion into wedges.
In a small jar or bowl, whisk together the olive oil, lemon juice, lemon zest, minced garlic, oregano, sea salt, and black pepper.
Add the vegetables to the bowl with the chicken and pour the lemon-herb dressing over the top.
Toss everything thoroughly with your hands or tongs until every piece is well-coated in the marinade.
Spread the mixture onto the prepared sheet pan in a single, even layer to ensure everything roasts evenly.
Roast in the center of the oven for 18 to 22 minutes, or until the chicken is cooked through and the vegetables are tender with slightly charred edges.