In a large skillet sauté onions in 1 tablespoon of olive oil, stirring frequently, for 2-3 minutes over medium heat. When onions become translucent, remove pan from the heat and leave them to cool a little.
In a large bowl, combine ground beef, ground pork, breadcrumbs, egg yolks, allspice, nutmeg, salt, pepper and cooked onion. Using a wooden spoon or clean hands, mix until well combined.
Roll the mixture into 1 ¼-to-1 ½-inch meatballs (ice cream spoon helps to measure the amount needed for one meatball), forming about 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 to get rid of any excessive fat.
To make the gravy, melt butter in the skillet. Whisk in flour until lightly browned, about 1 minute.
Gradually whisk in beef broth and cook, whisking constantly, until slightly thickened, about 1-2 minutes. Stir in sour cream; season with salt and pepper.
Serve immediately, garnished with parsley, and with cooked fresh pasta.