In a large bowl, combine the ground beef, ground pork, egg, breadcrumbs, onion, milk, salt, pepper, nutmeg, allspice, Dijon mustard, and parsley. Mix until just combined. Don't overwork it.
Using a 1.5-inch cookie scoop, portion out the meatballs and place them on a baking sheet.
Heat a large skillet over medium-high heat and brown the meatballs for 1-2 minutes per side. You're not cooking them through, just building a crust so they hold their shape in the slow cooker.
Transfer the meatballs to the slow cooker.
Whisk together the melted butter, beef broth, Worcestershire sauce, Dijon mustard, garlic powder, onion powder, salt, and pepper. Pour over the meatballs, cover, and slow cook on high for 3-4 hours or low for 5-6 hours.
About 15 minutes before serving, remove the meatballs from the slow cooker temporarily. Whisk the cornstarch slurry into the gravy, then whisk in the sour cream until smooth and starting to thicken. Taste and adjust seasoning.
Gently return the meatballs to the sauce. Switch the slow cooker to warm and let the sauce thicken uncovered for the last 10-15 minutes.
Serve over mashed potatoes or cooked egg noodles with extra parsley on top.