In a large bowl, soak breadcrumbs in milk for 1 to 2 minutes.
Add ground meat, grated onion, egg, salt, pepper, allspice, and nutmeg to the bowl; mix gently until just combined.
Form the mixture into small meatballs, about 1½ tablespoons each.
Set the Instant Pot to Sauté mode and heat olive oil.
Brown meatballs in batches, then remove and set aside.
Add butter to the pot, then pour in beef broth, Worcestershire sauce, and Dijon mustard. Scrape the bottom to deglaze.
Return meatballs to the pot, seal the lid, and cook on High Pressure for 5 minutes.
Allow a natural pressure release for 5 minutes, then quick release any remaining pressure.
Set Instant Pot to Sauté. Mix cornstarch with cold water and stir into the sauce; simmer 2 to 3 minutes until thickened.
Stir in heavy cream and heat gently until sauce is smooth and creamy. Adjust seasoning if needed.
Serve meatballs hot over mashed potatoes, egg noodles, or rice. Garnish with chopped fresh parsley if desired.