Toss the chicken, red onion, and bell peppers with the olive oil, lime juice, and fajita seasoning and let rest for 30 minutes. I like to do this in a ziploc bag for easy clean up. This step can be skipped if you are short on time.
Preheat the oven to 400 degrees. Spray a baking sheet with cooking spray. You can also cover with foil for easier clean up. Spread the chicken and vegetables in a single layer on the baking sheet in a single layer.
Place in the oven and cook for 10-12 minutes, until the chicken is just about cooked through and the vegetables are tender-crisp. Turn the oven up to broil to add some char and crispiness to the chicken for the last 2 minutes of cooking. One important note, the peppers and onions will still have some bite and texture when cooked this way. If you prefer a softer vegetable, place them in the oven 10 minutes before adding the chicken.