1. Preheat oven to 300 degrees Fahrenheit.
2. Place the roast into a roaster and scatter the brown sugar over it, patting it down. Pour cranberry sauce around the roast and sprinkle onion soup mix and sliced onions over the sauce. Bake in preheated oven for about three hours until soft.
3. Cool completely and then slice thinly. Reheat with sauce before serving.