Marinate the chicken: Place chicken in a nonporous glass dish or bowl. Pour teriyaki marinade over chicken and toss to coat. Cover and refrigerate for 1 to 2 hours.
Preheat the oven to broil.
Prep your veggies.
Melt butter in the same skillet over medium-high heat. Sauté onion, bell pepper, and mushrooms for about 3 to 5 minutes. Set onion mixture aside.
Remove chicken from the marinade and dry. Sauté chicken in the cast iron skillet until 170°-175°
Top baked chicken with onion mixture and top each breast with a slice of cheese. Broil until cheese is melted and bubbly, another 10 to 15 minutes.