Preheat oven to 300 degrees F (150 degrees C). Place a large sheet of heavy-duty aluminum foil in a shallow roasting pan.
Place rump roast in the roasting pan. Pour mushroom soup over the roast and sprinkle with onion soup mix.
Whisk wine and xanthan gum together in a small bowl; pour over the roast and top with mushrooms.
Fold the foil over the roast and crimp edges together to seal.
Bake in preheated oven for 4 1/2 hours. Let stand 10 minutes before serving.
This recipe does not contain many non-starchy vegetables. To maximize your results, it is highly recommended that you eat at least three servings of non-starchy vegetables via a SANE side dish or green smoothie along with this main dish.