Place a large sauté pan over medium heat. Add the butter and oil. Once the butter has melted, add the mushrooms.
Allow the mushrooms to sear for 5 minutes, stirring to flip them over. Then add the garlic and salt and pepper to taste. Allow the mushrooms to sear another 5 minutes to develop a rich caramelized color.
Stir in the fresh thyme leaves and sherry. Lower the heat to medium-low and simmer for approximately 10 more minutes, stirring on occasion. The goal is for the mushrooms to absorb the sherry, leaving only a small amount of moisture in the pan.
Taste, then salt and pepper again if needed. Serve warm over meat or as a side dish!