Place the butter in a skillet (I use cast iron) heated to medium high. When the butter melts, add the mushrooms and onion. Cook, stirring often, 3 minutes. Add the garlic; cook, stirring constantly, 30 seconds. Add the steak. Turn heat to high, cook, stirring often, untl beef is browned/seared well. Stir in 1/2 cup of the bouillon mixture, 1/4 cup sherry and the pepper. Turn heat down to low and let simmer about 7-10 minutes - depending on how you want your steak cooked.