Thoroughly the clean the mushrooms and slice them into thick slices (approximately 1/2-3/8 inch thick).
Heat the butter and the olive oil over medium high heat. Once the butter if fully melted and the oil is hot, slowly add the mushrooms to the pan and make sure not to over crowd the pan (you may have to cook in multiple batches depending on the size of the your pan).
Let the mushrooms cook in the pan for 3-5 minutes without stirring them. Then gently stir the mushrooms and cook for 2-3 more minutes until the mushrooms are browned on both sides and are the tenderness that you prefer. The cook time will vary based on the size of your mushrooms.
Season the mushrooms with the salt and pepper, serve warm and enjoy!