Bring the vinegar to a boil in a small nonreactive saucepan and cook, stirring occasionally, until thickened and reduced to about 3/4 cup, about 30 minutes. Stir in the honey and season with salt and pepper.
This is the type of soup that, at first glance, might seem a little…unexciting. But you’re underestimating the power of mushrooms, which do the heavy lifting.