In a large bowl, combine the ground beef, ground pork, breadcrumbs, egg, milk, allspice, nutmeg, salt, black pepper, and parsley. Mix gently with your hands until just combined. Roll the mixture into about 24 meatballs, roughly 1.5 inches each.
Set the Instant Pot to saute mode and melt the butter. Working in batches, sear the meatballs until browned on all sides, about 4 minutes per batch. Remove the meatballs to a plate as they finish.
Sprinkle the flour into the pot and stir it into the melted butter and drippings, cooking for 1 minute. Slowly pour in the beef broth while whisking constantly to prevent lumps. Stir in the worcestershire sauce and dijon mustard.
Carefully return all the meatballs to the pot in a single layer. Lock the lid and set the valve to sealing. Pressure cook on high for 5 minutes.
When the cook time is complete, do a quick pressure release by carefully switching the valve to venting. Remove the lid once the pin drops.
Gently stir in the sour cream until the sauce is smooth and creamy. Set to saute mode and let it warm through for 1 to 2 minutes without letting it boil.
Serve the meatballs and creamy sauce over cooked egg noodles, garnished with fresh parsley.