In a large mixing bowl, combine the ground beef, worcestershire sauce, breadcrumbs, eggs, allspice (if using,) onion powder, salt, and pepper. Mix until just combined.

Using a 1 tablespoon cookie scoop, portion the meat mixture and roll into a ball.

Heat a skillet over medium-high heat until hot. Add 1 tablespoon olive oil and then part of the meatballs, browning on all sides. It is not necessary to cook the meatballs all the way through. Transfer the meatballs to the slow cooker and continue browning the remaining meatballs in the remaining olive oil until they are all browned.

With all of the meatballs resting in the crock of the slow cooker, add the butter to the skillet to melt. Whisk in the flour, allowing it to cook for about 2 minutes.

Slowly whisk in the beef broth until the gravy is thick.

Remove the skillet from the heat and stir in the sour cream until blended.  Pour the gravy over the meatball in the slow cooker.

Cover and cook on low for 4 hours or high for 2 hours.

Gently toss the meatballs in the gravy. Sprinkle with chopped parsley and serve over egg noodles or mashed potatoes.