Grate 1 small onion into a bowl, keeping its juice. Remove the crusts from 2 slices of white sandwich bread, tear into pieces and mix with the onion. Leave for 1 minute.
Add 300g of ground beef, 300g of ground pork, 1 egg, ¼ teaspoon of nutmeg, ¼ teaspoon of ground allspice, ¼ teaspoon of black pepper, and ¾ teaspoon of salt. Mix well and roll heaping spoonfuls into balls.
Heat 1 tablespoon of oil in a large skillet over medium-high heat. Brown the meatballs in batches for about 3 minutes; lift out while still undercooked inside.
Discard excess fat. Melt 40g of unsalted butter over medium heat. Stir in 3 tablespoons of all-purpose flour and cook for 1 minute.
Gradually whisk in 2 cups of low-sodium beef stock until smooth. Return the meatballs and their juices. Simmer for 8-10 minutes, until the gravy thickens and the meatballs reach 160°F / 71°C inside.
Stir in ½ cup of heavy cream and simmer for 2 minutes. Season with salt and pepper.
