Set a 6-qt 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.
For the meatballs
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.
Notes
Keep your hands wet. When forming the meatballs, keep your hands slightly wet to keep the mixture from sticking to your hands, ensuring perfectly round, smooth meatballs.
Make ahead of time. The meatballs can be assembled up to 1 day in advance and placed in the fridge until ready to use. The uncooked meatballs can also be stored in the freezer for up to 1 month (once flash freezing).
Make a double batch to freeze. Make a big batch of meatballs, doubling the recipe, to have a stash ready in the freezer to cut weeknight dinner prep to just a few minutes.
Deglaze with beef stock. Deglaze the bottom of the Instant Pot with beef stock, lifting up the browned bits (fond) to incorporate all the rich, caramelized flavors to the gravy while also preventing the dreaded burn error.
Chicken stock also works. Unsalted chicken stock works well in place of the beef stock.
Serve with a starchy base. Serve the meatballs with a starchy base such as mashed potatoes or cooked egg noodles to sop up the rich, creamy gravy.