In the same skillet, add the mushrooms and sauté until browned, stirring occasionally, 2 to 3 minutes. Add the peppers and onions, and sauté for 2 to 3 minutes. Add the spaghetti sauce and mix well. Add back the chicken (with any juices). Simmer over medium-low heat until the chicken is cooked through, about 20 minutes. Chicken is done when it feels firm to the touch and a meat thermometer inserted in the center reaches 170°F.