Add in one more tablespoon of oil and your butter. Once your butter is completely melted add in your mushrooms. Sauté them for about five minutes until they have softened and caramelized. Then set aside.
If needed, add a tablespoon of oil. Add in your onions and sauté for about three minutes until they soften and caramelize. Add in your garlic and stir for another minute to release the flavors.