Heat 1 tablespoon olive oil in a large skillet over medium heat. Add onion and cook, stirring frequently, until onions have become translucent, about 2-3 minutes. In a large bowl, combine ground beef, ground pork, panko, egg yolks, allspice, nutmeg and cooked onion; season with salt and pepper, to taste. Using a wooden spoon or clean hands, stir until well combined. Roll the mixture into 1 1/4- to 1 1/2-inch meatballs, 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-lined plate. 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, to taste. Stir in meatballs and cook, stirring occasionally, until heated through and thickened, about 8-10 minutes. Serve immediately, garnished with parsley.