Mix the Meatballs: In a large bowl, combine the ground beef, ground pork, egg, breadcrumbs, onion powder, garlic powder, nutmeg, allspice, salt, and pepper. Mix gently until just combined—avoid overmixing so the meatballs stay tender.
Shape the Meatballs: Use a cookie scoop or your hands to form the mixture into 1 to 1½-inch meatballs. Arrange them on a parchment-lined baking sheet.
Broil to Brown: Set your oven to broil on high. Broil the meatballs for about 6 minutes, turning halfway through, until lightly browned on all sides. (They don’t need to be fully cooked—just seared for flavor.)
Transfer to Crockpot: Place the browned meatballs in the bottom of your slow cooker.
Make the Sauce: In a medium saucepan over medium heat, melt the butter. Whisk in the flour to form a roux, cooking for 1–2 minutes. Gradually whisk in the bone broth until smooth. Stir in the Dijon mustard, Worcestershire sauce, salt, and pepper. Simmer for a few minutes until thickened.
Cook: Pour the sauce evenly over the meatballs in the crockpot. Cook on Low for 5–6 hours or High for 2½–3 hours, until the meatballs are cooked through and tender.
Finish the Sauce: Once the meatballs are cooked, stir in the sour cream until smooth. Taste and adjust seasoning as needed.
Serve: Sprinkle with fresh parsley and serve over cooked egg noodles or shell pasta.