Melt butter over large nonstick skillet over medium-high heat. Add mushrooms and onions. Cook until the mushrooms and onions are soft, about 20 minutes, stirring every few minutes allowing the vegetables to brown.
Add balsamic vinegar and garlic salt. Reduce heat to medium, cooking for an additional 5 minutes or until the mixture thickens. Serve with grilled strip steaks.
Recipe by Mom to Mom Nutrition at https://momtomomnutrition.com/food-and-recipes/grilled-strip-steaks-mushrooms/