Preheat the oven to 500°F (260°C). Line a large baking sheet with parchment paper or foil and lightly spray with cooking spray.
Chop the bell peppers, red onion, zucchini, and broccoli into large, uniform pieces so they cook evenly.
Cut the chicken breasts into large cubes on a separate cutting board or plate.
Place the chopped vegetables and cubed chicken on the prepared baking sheet. Add the minced garlic, olive oil, and ranch seasoning over the top.
Toss everything on the sheet pan until the chicken and vegetables are evenly coated with oil, garlic, and ranch seasoning.
Bake in the preheated oven for 15 to 20 minutes, or until the chicken is cooked through and the vegetables are lightly charred.
Remove from the oven and serve immediately with rice, tortillas, or on its own.