In a 12-inch straight sided saute pan over medium heat, melt 1 tablespoon of the butter. Add the onion and a pinch of salt and sweat until the onions are soft. Remove from the heat and set aside.
In a large bowl, combine ground beef, ground pork, breadcrumbs, eggs, garlic, Worcestershire sauce, parsley, allspice, nutmeg and cooked onion, and salt and pepper. 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 30 meatballs.
Add 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 in which the meatballs were browned. Be sure to scrap up the brown bits from browning the meatballs. 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 and heavy cream and 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.