Add chopped mushroom stems and cook for an additional 3 minutes.
Chop the mushroom stems. In a skillet add the butter and olive oil and heat until butter melts over medium heat. Add the chopped onion and garlic and cook until onion is translucent.
Clean the mushrooms using a mushroom brush and then gently remove the stems without breaking the mushrooms. Place the mushrooms stems side down on a baking pan and bake them for 10 to 15 minutes until the water leaks out of them. Soak up any additional moisture with paper towels.