Place mushrooms in a flat-bottom pan. Pour beef broth over, add sage, stir, then bring to a boil. Don’t worry if your mushrooms are not completely covered by the broth. Mushrooms make their own juices while cooking.
Once it’s boiling, reduce to medium heat. Add white wine and cook (uncovered) for 20 minutes.
At the last minute of cooking, dissolve cornstarch with cold water in a small bowl, then stir into the pan, until the sauce is thickened.