Heat oil in large skillet or wok over medium-high heat until hot. Add chicken strips and onion; cook and stir 3 minutes. Add bell pepper; cook and stir 2 to 3 minutes or until bell pepper is crisp-tender and chicken is no longer pink. Stir in salsa. Remove from heat.
Spread bottom half of each bun with warm refried beans. Spoon chicken mixture onto each. Sprinkle with cheese and cilantro. Spread top halves of buns with sour cream; place over cheese and cilantro.