In a medium saucepan over medium heat, melt 2 tablespoons of the butter. Add the mushrooms slices and sauté until golden brown, about 4 minutes.
Add the garlic and sprinkle with salt and pepper, and cook one minute more. Remove and let cool, then use a fork to mix the mushrooms into the remaining butter. Place the butter on a piece of waxed paper and wrap it up, then roll into a log. Refrigerate until firm.
Tenderloin Steak
Allow the steaks to come to room temperature for at least 30 minutes before cooking. Season the steaks on both sides with salt and pepper.
Heat a large skillet over medium high heat. Once hot, add the oil and let heat for about 1 minutes. Add the steaks and sear 3 to 5 minutes per side, depending on thickness. Remove from heat and let rest in the pan another 3 minutes or so, until the internal temperature reaches 140F to 150F.
Serve with a large dollop of Mushroom Garlic Compound Butter on top.