This was a never before written down recipe handed down from my great grandmother who lived in Berks county, Pennsylvania. I grew up in Lancaster county. My mother makes this every year with her Thanksgiving and Christmas dinners. It's great with poultry, or even beef, with your favorite meat gravy. It makes a great substitute for stuffing or dressing on the side. My grandfather affectionately described potato filling as "mashed potatoes with a whole lot of good stuff in it." Mix in left over turkey and gravy leftovers or use it for shepherd's pie. It's uses are endless!
Preheat oven to 375 degrees F. Boil potatoes in salted water for 1 hour or until they can easily be pierced with a fork. Meanwhile, melt 1 tablespoon of butter, add onions, and saute until just soft, add bread crumbs and stir over heat until lightly toasted. Melt remaining butter separately, and set aside.
Transfer potatoes to a large mixing bowl and add torn bread, onions and remaining ingredients. Whip together with a hand mixer on medium speed until mixture is almost smooth. Mixture should be a fluffy, mashed potato consistency, just a little bit more dense and rich. Transfer potato mixture to a glass 8x8" square baking dish. Drizzle remaining 2 Tablespoons butter over filling.
Bake, uncovered, 30-35 minutes or until filling has formed a light crust. Serve.