Soak breadcrumbs in milk for 5 minutes, then combine with beef, pork, egg, nutmeg, allspice, and salt.
Roll into small 1-inch balls.
Brown meatballs in batches in butter in a large skillet over medium-high heat, about 4 minutes per batch; set aside.
In the same skillet, whisk flour into the drippings, then slowly add beef broth, whisking constantly.
Add heavy cream and simmer 3-4 minutes until thickened.
Return meatballs to the pan and simmer in the gravy for 5 minutes.
Transfer to a serving dish or slow cooker set to warm.