In a large bowl, add the panko bread crumbs and milk; stir them together and allow to sit for 5 minutes. Next, crack in the eggs and whisk until combined. Then add the pork, beef, salt, black pepper, nutmeg, cardamom and allspice. Mix the meatballs until just combined, being sure not to over-mix. Using a tablespoon or a small ice cream scoop, form the meatballs and set aside, you should end up with about 30 meatballs.
To a medium skillet, set over medium heat, add the oil. When the oil is hot, add the meatballs, searing them on both sides, for about 2 to 2 minutes. You’ll definitely need to do this in batches. Remove the meatballs and set aside.
Add the butter and when melted add the flour. Stir the two together, cooking the roux until the flour mixture is smooth. Pour in the beef stock and bring to a simmer until thickened, about 5 minutes. Mix in the sour cream until smooth. Add the meatballs to the pan and cook for about 10 minutes. Serve alongside creamy mashed potatoes. These can be made the day before, kept in a fridge-safe container and then reheated (you may need to add a splash of water to get the gravy to the right consistency!).