In a medium-sized bowl, combine all of the marinade ingredients until dissolved. Set aside.
Trim chicken breasts and pierce each one 6–8 times with a fork. Nestle chicken into the bowl of the marinade and stir to coat each piece. Cover with plastic wrap or seal the plastic bag and refrigerate at least 4 hours and up to overnight.
Preheat the oven to 400 degrees.
Spray a 9" x 13" baking dish with cooking spray. Scatter onion and bell peppers across the bottom of the dish. Remove chicken breasts from the marinade and place on top of the onions and peppers.
Add mushrooms into the remaining marinade, stir to coat then transfer the mixture to the baking dish. Settle the mushrooms in around the chicken.
Sprinkle the casserole with Parmesan cheese and bake for 25–35 minutes depending upon the thickness of chicken, until chicken is cooked all the way through. If you’d like a little more color on your chicken, just place the entire dish under the broiler for 2–3 minutes to crisp up the top.
Remove dish from oven and transfer chicken to a serving platter. Season pan juices and veggies with salt and pepper then carefully pour it over the chicken breasts. Serve with a little Parmesan cheese on the side for sprinkling.
Notes
If you want to add more color to the finished dish, place it under the broiler for a few minutes to crisp it up.