Melt the butter in a saucepan over MED-HIGH heat. Add the mushrooms and garlic. Sauté it for 8–10 minutes, until the mushrooms are cooked and most of the water has cooked off.
Sprinkle in the flour and stir as you slowly pour in the beef stock.
Stir the gravy constantly while you bring it to a boil over MEDIUM heat, then turn down the heat and let it simmer until it thickens, 10–15 minutes.