Sprinkle chicken pieces with salt and pepper. In a large skillet, heat oil over medium-high heat. Cook chicken in hot oil until browned.
In a 4 to 5-quart slow cooker, combine onion, bell pepper and mushrooms. Place chicken pieces on top of onion mixture. Sprinkle chicken with garlic powder. Pour tomatoes over chicken. Cover and cook on LOW for 7 hours.
Remove chicken from slow cooker. Skim fat from surface of liquid in slow cooker, if desired. Serve chicken and vegetable mixture over hot cooked pasta, potatoes or rice.
Cook's Note: We used 2 skinless chicken breasts and 3 skinless chicken thighs.