Add the beef broth and Worcestershire sauce to a 6 quart slow cooker.
In a large mixing bowl, combine beef, pork, breadcrumbs, eggs, allspice, nutmeg, and onion powder. Add salt and pepper, to taste.
Mix until smooth and even colored and textured. Scoop tablespoonfuls out and roll them into balls, placing them in the bottom of the slow cooker. If a second layer needs to be made, start near the outside of the slow cooker.
Cover and cook meatballs thoroughly, 2 hours on high, about 6 hours on low.
Meanwhile, combine flour and butter in a small bowl to form a paste. Pinch off into tiny balls and set aside.
Transfer the cooked meatballs to a large bowl, leaving the liquid in the slow cooker. Add the flour balls to the cooking liquid and whisk to evenly incorporate. Stir in the sour cream. Return meatballs and any juice to the slow cooker, stirring to coat the meatballs in the sauce.
Finish on high, uncovered, cooking for 30 minutes. This allows the sauce to thicken.
Garnish with parsley, and serve. They're delicious over egg noodles or with mashed potatoes.