Mix breadcrumbs and milk in a large bowl and let soak for 1 to 2 minutes.
Add ground meat, onion, egg, salt, pepper, allspice, and nutmeg to the breadcrumb mixture and gently combine.
Form the mixture into small meatballs, about 1½ tablespoons each.
Set the Instant Pot to Sauté mode and heat olive oil.
Brown the meatballs in batches until golden; they do not need to cook through. Remove and set aside.
Add butter to the pot and then pour in beef broth, Worcestershire sauce, and Dijon mustard, scraping the bottom to deglaze.
Return the meatballs to the pot.
Seal the Instant Pot lid and cook on High Pressure for 5 minutes.
Allow natural pressure release for 5 minutes, then perform a quick release.
Switch back to Sauté mode and stir the cornstarch mixed with cold water into the sauce.
Simmer for 2 to 3 minutes until thickened.
Stir in heavy cream and heat gently until the sauce is smooth and creamy.
Adjust seasoning to taste and serve meatballs hot over mashed potatoes, egg noodles, or rice, garnishing with chopped parsley if desired.