In a medium skillet, heat oil over medium-high heat until shimmering. Add mushrooms and onion; sprinkle with salt and stir to combine.
Reduce heat to medium. Cover and cook until liquid is released from mushrooms, about 5 – 7 minutes. Remove cover and continue to cook until liquid has evaporated and mushrooms and onions are softened and browned, about 3 -5 minutes.
Add balsamic vinegar and stir until mushrooms and onions are coated evenly and the vinegar is heated through.
Serve as an accompaniment to steak or chicken or as a healthy side dish (vegan and gluten free).