In large saucepan, heat half the oil over medium-high heat. Brown chicken on all sides, about 8 min. Transfer chicken to a plate.
Add remaining oil to saucepan and cook onion and pepper until starting to soften, about 2 min. Stir in tomatoes, chili powder and cumin. Add chicken back into saucepan. Bring to a boil, then reduce to a simmer. Loosely cover and cook, stirring occasionally, until sauce is slightly thickened and chicken is cooked through, about 10 min.
Stir in kidney beans and cook, stirring occasionally, approx. 5 min., until heated through. Top with green onions, sour cream and Cheddar cheese to serve, if desired.