Melt 1 tbsp. butter in a medium skillet over medium-high heat. (If you’ve just cooked your steak on the stove, use the same skillet. The pan scrapings will enhance the sauce.) Add mushrooms, and saute until brown and juices are released. Add garlic, a pinch of salt, and a few grinds of pepper. Saute about two minutes longer, being careful not to let the garlic burn.
Stir in wine, and bring to a boil. Continue cooking at a low boil until sauce is reduced by half, about 10 minutes.
Turn heat to low, and stir in remaining 1 tbsp. butter. Turn off heat, and stir in any juices that have accumulated from the resting steak. Spoon sauce with mushrooms over cooked steak. Swoon.
I would not recommend preparing this sauce in an iron skillet. Use stainless steel if you have it.