Grill Kansas City Strip Steaks over a medium fire to desired doneness. Season the steaks after cooking. While steaks grill, melt half the butter in a large saute pan and saute the red onions and mushrooms until they become limp. Add the Dijon Mustard and saute for another 30 seconds, stirring, then add the basalmic vinegar, stir well and cook until almost evaporated. Add the beef stock and cook 1 minute, stir well. Remove pan from the heat and swirl in remaining butter into the sauce. Add salt/pepper to taste. Spoon sauce on warm plates and place the steaks on top.