Meanwhile, in 12-inch skillet, heat oil over medium-high heat until hot. Add chicken; cook and stir 2 to 4 minutes or until chicken is beginning to brown. Stir in onions, mustard and garlic. Cook until garlic is softened. Stir in tomatoes until blended. Cook 2 to 3 minutes, stirring occasionally, until chicken is no longer pink in center. If necessary, drain off any liquid.