In a large skillet, brown pork in butter over medium-high heat. Transfer meat to an ungreased shallow baking dish. Sprinkle with salt and pepper and set aside.
In the pan of pork drippings, saute the onion and mushrooms until tender.
In a bowl, combine flour and broth until smooth. Add to skillet.
Bring the skillet to a boil. Cook and stir for about two minutes. Pour over meat.
Cover and bake at 325 F for 1 hour. Let stand before slicing.