In a large bowl, combine the panko breadcrumbs and milk, then let the mixture sit for a few minutes until softened. Add the ground beef, ground pork, egg, finely chopped onion, nutmeg, allspice, salt, and pepper, then gently mix until just combined.

Shape the mixture into evenly sized meatballs using lightly oiled hands. Place the meatballs onto a tray and prepare a skillet with olive oil over medium heat.

Brown the meatballs in batches until deeply golden on all sides, being careful not to overcrowd the pan. Transfer the browned meatballs to a plate while finishing the remaining batches.

In the same skillet, melt the butter and whisk in the flour to create a smooth roux. Cook for about a minute, then gradually whisk in the beef stock until the gravy becomes smooth and thickened.

Return the meatballs to the skillet and spoon the gravy over them evenly. Let the meatballs simmer gently in the sauce until fully cooked through and tender.

Reduce the heat to low and stir in the sour cream until the gravy becomes creamy and silky. Avoid boiling after adding the sour cream to maintain a smooth texture.

Finish the dish with freshly chopped parsley for brightness and color. Serve hot with mashed potatoes, egg noodles, or your favorite side dish.