Place beef slices on a cutting board and gently pound using a meat tenderizer.
Over each slice, spread a thin layer of mustard, season with pepper, top with bacon, onions, and a dill pickle.
Roll up each Rouladen lengthwise, around the pickle, and secure with toothpicks.
Heat butter in a pan and brown each roll. Place rolls in a roasting dish, add broth, canned mushrooms, and ¼ cup pickle juice. Roast 90-120 minutes or until fork-tender.
Remove Rouladen from the juices, set on a plate and cover.
Heat juices over medium heat until boiling. Combine xanthan gum with an equal amount of water. Slowly add the mixture to the boiling broth while whisking until it reaches the desired thickness (You may not need all of it).
Serve Rouladen over mashed cauliflower with gravy.