Preheat oven to 400°F (200°C) and line a baking sheet with parchment paper.
In a small bowl, whisk together olive oil, garlic, lemon juice, oregano, salt, and pepper.
Place chicken breasts on one side of the sheet pan and brush generously with the lemon-garlic mixture.
In a separate bowl, toss broccoli, bell peppers, and onion with olive oil, salt, and pepper. Spread them on the other side of the pan.
Bake for 20–25 minutes, or until the chicken reaches an internal temperature of 165°F (74°C) and vegetables are tender.
Garnish with lemon slices and serve immediately.