Grate the onion (1), scrape it and its juices into a bowl with the bread (2) and mix - the bread should go soggy (add a splash of milk if not). Soak 1 minute.
Add beef (300 g), pork (300 g), egg (1), nutmeg (1/4 tsp), allspice (1/4 tsp), pepper (1/4 tsp) and salt (3/4 tsp); mix well. Roll into 25-30 heaped-tablespoon meatballs.
Heat 1 tbsp oil (1 tbsp) over medium high. Brown meatballs in 2 batches, all over but raw inside (~3 min each). Set aside. Pour off excess oil; lower heat to medium.
Melt butter (40 g) in the same pan, stir in flour (3 tbsp) and cook 1 min. Gradually whisk in the beef broth (500 ml) until smooth.
Once simmering, return meatballs and juices. Keep at rapid simmer 8-10 min until gravy thickens, stirring occasionally. Stir in sour cream (120 ml), simmer 2 min more. Serve over mashed potato.