Rinse pepper and chicken; pat dry. using separate cutting boards for chicken and vegetables, slice chicken, pepper and onion into strips.
In a nonstick skillet, heat oil on medium heat and stir fry chicken until slightly golden, about four to six minutes. Increase heat to medium-high and add pepper, onion and chili powder to skillet. Continue to cook, stirring frequently until vegetables are tender but still firm, about another two to three minutes.
Rinse and chop tomato; add to chicken mixture. Spread mixture over tortilla(s), sprinkle with cheese, top with sour cream and serve.