Clean and slice the mushrooms. Slice the red onion and finely chop the garlic.
Put a big skillet on medium heat and add the oil. Once the oil is hot, add the onion and garlic. Sauté for about 3 minutes, until the onion just start to get brown.
Add the mushrooms, salt and pepper. Sauté for 5 minutes on medium heat, stirring constantly. The mushrooms will get darker and tender.
After 5 minutes, add the balsamic vinegar and sauté for another minute.
Take off heat and transfer to a serving bowl/storage container. The balsamic glazed mushrooms pair really well with sunny side up or scrambled eggs.