In 10-inch skillet, heat oil over medium-high heat. Add chicken; sprinkle with salt and pepper. Add onion slices; cook 4 to 5 minutes, turning chicken once, until chicken is brown on all sides.
Add beer; heat to boiling. Reduce heat to low; cover and simmer about 10 minutes or until chicken is fork-tender and juices run clear.
Stir in bell peppers. Increase heat to medium; cook uncovered 5 minutes longer, stirring occasionally, until peppers are crisp-tender.