For sauce: Melt butter and sate shallots with garlic for one minute. Add mushrooms and cook for 5 minutes. Add beef stock, brandy, cream and mustard. Bring to a boil and simmer about 10 minutes until reduced by half. For Steaks: Combine mustard, olive oil, salt and pepper. Coat both sides of steak with the mixture. Grill on medium high heat for 5 minutes per side or until desired doneness. Drizzle with mustard sauce.