Add the ground beef, pork rind crumbs, Worcestershire sauce, garlic, egg and salt and pepper to taste to a large mixing bowl. Knead with your hands until the mixture is blended well.
Form the mixture into 6 evenly sized patties.
Heat the oil in a skillet over medium high heat.
Place the patties into the skillet, and sear evenly on both sides. Remove, and place into the bottom of the Crockpot.
Melt the butter in a skillet, and add in the onions and mushrooms. Saute until softened.
Stir in the garlic, and saute for 30 seconds.
Pour in the beef broth, and simmer for 5 minutes.
Lightly sprinkle the xanthan gum into the skillet, whisking as you do.
Pour the gravy mixture over the patties in the Crockpot.
Cover and cook on high for 3-4 hours.