Heat oil in large skillet on medium-high heat. Add onion, bell pepper and celery; cook and stir 2 minutes or until vegetables are tender-crisp. Add chicken; cook 5 minutes per side or until browned.
Mix mushroom gravy and milk in small bowl. Stir into chicken mixture. Bring to boil. Reduce heat to low; cover and simmer 12 minutes or until chicken is cooked through.