Step 1

In a medium skillet over medium-high heat, heat oil until shimmering. Add mushrooms, onions, and 1/4 teaspoon salt and stir to combine.

Step 2

Reduce heat to medium, cover, and cook until liquid is released from mushrooms, about 5 minutes. Remove cover and continue to cook until liquid has evaporated and mushrooms and onions are softened and browned, about 5 minutes longer.

Step 3

Stir in balsamic vinegar and stir until mushrooms and onions are coated evenly and the vinegar is heated through, about 2 minutes.