Combine first 4 ingredients in a small bowl, stirring well with a whisk.
Heat oil in a large nonstick skillet over medium-high heat. Add chicken, and cook 3 minutes; turn and reduce heat to medium- low. Cover and cook 12 minutes or until chicken is done. Remove chicken; keep warm.
Remove pan from heat. Add mustard mixture to drippings in pan; stir well. Spoon mustard sauce over chicken; top with green onions.