Preheat the oven to 300° F.
Cook the potatoes in boiling salted water until tender.
Drain, reserving the water.
Crumble the bread into the bowl of an electric mixer.
Add 1/2 cup of the potato water and beat until mushy.
Add the potatoes and beat until well blended.
Add the hamburger, sausage, onion, eggs, salt, pepper, allspice, and nutmeg.
Beat for 4 minutes.
Add a little more potato water if the mixture seems too dry.
Heat the butter and oil in a large heavy skillet.
Roll half tablespoonfuls of the mixture between palms of hands and drop them into the hot fat.
Shake the skillet so that the meatballs brown lightly on all sides.
Transfer the browned meatballs to a roasting pan.
Repeat the process until all of the meatball mixture has been used.
Add the Beef Stock to the roasting pan.
Cover with aluminum foil.
Bake at 300° F for 1 hour.
Transfer the meatballs to a chafing dish.
Add 1 cup of the cooking liquid to the Sweet and Sour Sauce.
Pour the sauce over the meatballs.
Serve hot.