In a large cast iron skillet, heat 1 tablespoon of olive oil over medium heat. Add the diced onion and cook, stirring often, until the onions are translucent, which should take about 2-3 minutes.

In a large mixing bowl, combine the ground beef, ground pork, Panko, egg yolks, allspice, nutmeg, and the cooked onion. Season the mixture with salt and pepper to your liking.

Using a wooden spoon or your hands, mix everything together until well combined. Form the mixture into meatballs, each about 1 ¼ to 1 ½ inches in size, yielding approximately 24 meatballs.

Add the remaining tablespoon of olive oil to the skillet. In batches, add the meatballs and cook them until they are browned on all sides, which should take about 4-5 minutes. Once browned, transfer the meatballs to a plate lined with paper towels.

To prepare the gravy, melt the butter in the same skillet. Whisk in the flour and cook until it turns lightly brown, about 1 minute.

Gradually add the beef broth while whisking constantly, and continue to cook until the mixture thickens slightly, which should take about 1-2 minutes.

Stir in the sour cream and season with salt and pepper to taste. Add the meatballs back into the skillet and cook, stirring occasionally, until they are heated through and the gravy has thickened, about 8-10 minutes.

Serve the meatballs immediately, garnished with chopped parsley if desired.