Heat your gas or charcoal grill to high (Here's how to tell if it's high: Hold your hand about four inches above the cooking surface. After 2 to 3 seconds, the heat will force you to pull your hand away.)
Place the steak on the grill. Use these cooking times to cook your steak to medium: 2 to 3 minutes per side for a 1/2 inch thick steak, 4 to 6 minutes per side for a 1 inch thick steak, 6 to 9 minutes per side for a 1 1/2 to 2 inch thick steak.
While steak is grilling, heat oil in skillet over medium heat. When oil is hot, add mushrooms and garlic and cook, stirring, for 3 minutes. Add remaining ingredients, stir, and cook for another 3 minutes until heated through.
Take the steak off the grill. Let the meat rest 2 to 3 minutes to allow the juices to reabsorb into the steak. Pour creamy sauce over steak and serve.