Meatballs:

• 2 tablespoons olive oil, divided
• 1 onion, diced
• 1 pound ground beef
• 1 pound ground pork
• 1/2 cup panko bread crumbs
• 2 large egg yolks
• 1/4 teaspoon ground allspice
• 1/4 teaspoon ground nutmeg
• Kosher salt and freshly ground black pepper, to taste

Gravy:

• 1/4 cup unsalted butter
• 1/3 cup all purpose flour
• 4 cups beef broth
• 3/4 cup sour cream
• Kosher salt and freshly ground black pepper, to taste
• 2 tablespoons chopped fresh parsley leaves

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 to 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 to 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 to 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 to 10 minutes. Serve immediately, garnished with parsley.