Wash mushrooms and dry with a clean towel. Cut mushrooms into thick slices.
Melt butter in large frying pan on medium-high heat. Add the mushrooms in an even layer and let sit for 1-2 minutes.
Toss mushrooms to evenly color, adjusting heat if necessary. Season with salt black pepper and add the shallot. Stir and cook for 30 seconds. Add the optional Dry Sherry or Pernod, garlic, and thyme. Stir together for a few seconds, remove from the heat and serve.