Add mushrooms to reserved drippings; cook 6 min., stirring frequently to loosen browned bits from bottom of skillet. Add leeks; cook, stirring frequently, 6 min. or until tender. Add cream cheese; cook and stir 2 min. or until melted. Add thyme, and remaining mustard, pepper and garlic powder; mix well. Remove skillet from heat. Return chicken to skillet; cover with foil.