Preheat your oven to 400°F (200°C) and line a large rimmed sheet pan with parchment paper for easy cleanup.
Cut the chicken breasts into 1-inch bite-sized pieces and set aside.
Chop the broccoli into bite-sized florets, slice the bell pepper into thin strips, and cut the red onion into small wedges.
In a small mixing bowl, whisk together the extra virgin olive oil, the juice from half of the lemon, minced garlic, dried oregano, sea salt, and black pepper.
Place the chicken pieces and all the chopped vegetables onto the prepared sheet pan.
Drizzle the lemon-herb marinade over the chicken and vegetables, then toss thoroughly with your hands or tongs to ensure an even coating.
Spread everything out into a single layer, ensuring the pan isn't overcrowded so the ingredients roast rather than steam.
Thinly slice the remaining half of the lemon and scatter the rounds across the top of the mixture.
Roast in the center of the oven for 20 to 25 minutes, or until the chicken is cooked through and the vegetables are tender with slightly charred edges.