In a small bowl, combine the breadcrumbs and milk. Let them sit for a few minutes until the breadcrumbs absorb the milk.
In a large mixing bowl, add the ground beef, ground pork, soaked breadcrumbs, finely chopped onion, parsley, egg, salt, black pepper, allspice, and nutmeg.
Gently mix all the ingredients until well combined. Be careful not to overmix, as this can make the meatballs tough.
Shape the mixture into meatballs, approximately 1 to 1.5 inches in diameter. You should get about 24 meatballs.
Sear the Meatballs in the Instant Pot
Set your Instant Pot to "Sauté" mode and add a bit of oil.
Once the pot is hot, add the meatballs in batches, ensuring not to overcrowd the pot. Sear them until they are browned on all sides. Remove the meatballs and set them aside.
Pressure Cook
Add the beef broth and diced mushrooms to the Instant Pot liner and use it to deglaze the bottom of the pan, using the back of a spoon to remove any browned bits.
Add the meatballs back to the pot with the broth.
Close the Instant Pot lid and set it to "Pressure Cook" or "Manual" mode. Cook on high pressure for 8-10 minutes.
Once the cooking cycle is complete, allow for a natural pressure release for about 5 minutes. Then, carefully perform a quick pressure release.
Create The Sauce
Remove the Instant Pot lid.
Add the salt, pepper, heavy cream, and Worcestershire sauce to the cooked meatballs and broth.
Set the Pot to saute. Stir until the sauce starts to thicken and is warmed through.