In a large bowl, combine the beef, egg, mustard, pickles, onion, pepper, and Worcestershire sauce. Mix gently with your hands or a wooden spoon until just combined—do not overmix, as that can make the meatballs tough.
Use a medium cookie scoop or tablespoon to portion the mixture into 24 roughly 1-ounce meatballs. Roll into balls and place directly into the Crock-Pot.
Prepare The Sauce:
In a medium bowl, whisk together the ketchup, mustard, pickle juice, and vegetable broth until smooth and fully combined.
Combine And Cook The Dish:
Pour the sauce evenly over the meatballs in the Crock-Pot.
Cover and cook on high for about 3 hours, until the meatballs are cooked through and tender.
Finish The Sauce:
Use a slotted spoon to carefully remove the meatballs from the Crock-Pot and transfer them to a bowl or serving dish.
Strain the sauce that's left in the Crock-Pot through a fine-mesh sieve into a saucepan, removing any solids.
Bring the strained sauce to a gentle boil over medium heat. Sprinkle in the xanthan gum and whisk constantly until the sauce thickens and becomes sticky, about 2 minutes.
Serve The Dish:
Pour most of the sauce over the meatballs, reserving some for dipping on the side. Serve warm.