While Boneless Ribeye Steaks are grilling, melt 3 tablespoons of butter in a 12 inch non-stick skillet over medium-high heat and saute onions and mushrooms until they become limp. Whisk in the Dijon Mustard and reduce heat to low. Saute another 30 seconds, stirring, then add the balsamic vinegar, and cook mixture until liquid is almost gone. Add the chicken stock and cook another minute. Remove skillet from the heat and swirl in 2 tablespoons of butter. Add salt and pepper to taste. Spoon the sauce onto six warm plates and place the grilled steaks on top of the sauce.