Chop the mushrooms into small pieces, whatever size you like to eat. They will shrink when cooked. Chop the onion fine.

Heat a medium skillet over medium-low heat and add 1 tablespoon of olive oil. 

When oil is shimmering, add the onions and cook, stirring occasionally, until the onions begin to sweat and release some moisture.

Add the mushroom to the onions and sprinkle with salt and pepper. Cook until soft and very brown, stirring occasional, about 10 minutes.

Pour vinegar in the pan and scrape any browned bits off the bottom of the pan. Turn off the heat and taste for seasoning. Add salt, pepper, or more vinegar as needed.