Clean mushrooms and cut with a knife into similarly shaped thick pieces, if desired. Do not cut the mushrooms thin.
Add butter and olive oil to a pan. Heat over medium-high heat.
Once butter is melted and the mixture is hot, add mushrooms in small batches allowing room between mushrooms. Avoid crowding the pan. Allow mushrooms to cook, without stirring for 3-5 minutes.
Stir only occasionally allowing mushrooms to finish cooking completely for another few minutes. Add salt and pepper to taste. Cook time varies depending on the size of the mushroom. If the mushrooms give off a lot of liquid, keep cooking until the liquid has evaporated and the mushrooms are browned on both sides.
Serve warm over steak, in pasta, or as a side.
To Make Ahead: Mushrooms can be sauteed ahead of time and stored covered in the fridge for 3-4 day. Reheat in a skillet until warm.To Freeze: Freezing may change the texture of sauteed mushrooms. After cooking, freeze mushrooms flat on a cookie sheet. Once completely frozen transfer to a freezer-safe container. Thaw and reheat in a pan over medium heat.Deglaze: Add a few tablespoons of water, wine, stock, cream, or sherry to the pan after the mushrooms are done cooking. Keep the temperature up and stir until the liquid evaporates.Flavor variations:Garlic and herbs: Add last minute of cooking. Wonderful herbs to try are thyme, parsley, and rosemary.Shallots: Add minced shallots about halfway through the cook.Mushrooms and Onions: Add 1 large sliced onion and an additional tbs. of olive oil to the pan a few minutes before adding the mushrooms. Stir occasionally. Cook may take a few extra minutes. Add garlic and seasoning the last minute of cooking. You want the onions to be translucent, slightly browned, and soft. There should also be no excess liquid left in the pan.