Season chicken with salt and pepper. In 10-inch nonstick skillet, heat oil over medium-high heat until hot. Cook chicken in oil about 5 minutes, turning occasionally, until no longer pink in center. Remove chicken from skillet; set aside and keep warm. To same skillet, add onion; cook about 3 minutes, stirring occasionally, until tender. Remove onion from skillet, set aside. Reduce heat to medium.