Add steaks; cook 2 to 3 min. on each side or until evenly browned on both sides. Cook 3 to 4 min. on medium heat or until steaks are medium doneness (160F), turning occasionally.
Remove steaks from skillet; cover to keep warm. Discard drippings from skillet.
Add onions to skillet; cook and stir 1 min.
Add sherry; cook 1 min., stirring constantly to loosen browned bits from bottom of skillet. Stir in cream and mustard; cook 5 min. or until thickened, stirring constantly.