Put tinly sliced potatoes in a well greased 2 quart casserole. Add 1 cup milk, sprinkle with salt and pepper. Combine remaining 1/4 cup milk with bread crumbs, seasonings, butter and onions. Spread over top of potaotes. Cut tenderloin into half inch slices, pound flat. Place on top of bread dressing. Dot with butter. Cover and bake at 350 F. for 1 1/2 hours. Serves 4.