In a large bowl, combine ground beef, ground pork, Panko, milk, egg yolks, shallot, garlic, thyme, allspice and nutmeg; season with salt and pepper, to taste.
Using a wooden spoon or clean hands, stir until well combined. Roll the mixture into 3/4-to-1-inch meatballs, forming about 30-32 meatballs.
Set Instant Pot® to the high saute setting. Heat canola oil; working in two batches, add meatballs and cook until all sides are browned, about 3-4 minutes. Transfer to a paper towel-lined plate; set aside.
Whisk in flour until lightly browned, about 1 minute. Stir in beef stock, scraping any browned bits from the bottom of the pot; season with salt and pepper, to taste.
Return meatballs to the Instant Pot®. Select manual setting; adjust pressure to high, and set time for 8 minutes. When finished cooking, quick-release pressure according to manufacturer’s directions.
Set meatballs aside. Select high sauté setting; simmer until the sauce has reduced and thickened, about 4-5 minutes. Stir in sour cream; season with salt and pepper, to taste.
Serve immediately with meatballs, garnished with dill and parsley, if desired.
Serving Size 1