Now that I had a method for sautéing mushrooms that was not only efficient but also downright lean, I felt justified in lavishing the mushrooms with a glossy butter-based glaze.
Once they were well browned, I lowered the heat to medium and pushed the mushrooms to the sides of the pan to clear a space. In went a tablespoon of butter and some minced shallot and rosemary to lend the dish depth and fragrance, followed by red wine and cider vinegar to deglaze the pan.
I gave the contents a stir to evenly coat the mushrooms with the sauce, but on closer inspection I noticed that the mushrooms were coated with droplets of fat—a clear sign that simply tossing the butter-wine mixture with the mushrooms hadn't thoroughly emulsified the sauce.
The problem was that there was very little sauce compared to the volume of mushrooms in the pan, which made it difficult to vigorously stir the mixture and incorporate the butter. So, going forward, I added ½ cup of chicken broth after the deglazing liquid had evaporated, and I let the buttery broth simmer until it had reduced by about half and the fat had emulsified into the liquid.