Melt the butter in a large skillet over medium heat. Add the mushrooms and season with the salt and pepper. Cook undisturbed until the mushrooms are browned on the bottom, about 5 minutes. Flip the mushrooms and cook undisturbed until browned on the second side and tender, about 5 minutes more. Add the garlic and thyme and sauté until fragrant, about 1 minute. Serve immediately.