Heat 1 tablespoon olive oil in a large skillet over medium heat. Add onion, and cook, stirring frequently, until onions are translucent, about 2-3 minutes; set aside.

In a large bowl, combine ground beef, ground pork, bread crumbs, egg yolks, allspice, nutmeg and onion. Season with salt and pepper, to taste.

Stir mixture with a meat chopper or wooden spoon until well combined. Use clean hands to roll the mixture into meatballs about 1 1/2-in in size (It should make about 20-24 meatballs).

Add remaining 1 tablespoon olive oil to the skillet. Add meatballs, in batches, and cook until all sides are browned, about 4-5 minutes. Transfer to a paper towel-lined plate.

Melt butter in the skillet. Whisk in flour until lightly browned, about 1 minute. Gradually whisk in beef broth and cook, stirring constantly, until slightly thickened, about 1-2 minutes.

Stir in sour cream; season with salt and pepper, to taste. Stir in meatballs and cook, stirring occasionally, until heated through and thickened, about 8-10 minutes.

Serve immediately, garnished with parsley, if desired.