Cut steak into 4 pieces; sprinkle with steak seasoning. Grill steaks, covered, over medium heat or broil 4 in. from heat for 5-7 minutes on each side or until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°; medium-well, 145°).
Meanwhile, in a large skillet, saute mushrooms in butter until tender. Stir in the remaining ingredients. Bring to a boil; cook until liquid is reduced by about half. Serve with steaks.