Slice the beef very thinly against the grain. Place the beef in the bottom of a slow cooker.
In a medium bowl or glass measuring up, whisk together the brown sugar substitute, soy sauce, water, sesame oil, garlic, ginger, and red pepper flakes.
Pour over the beef. Place the lid on the slow cooker and cook on low for 4 to 6 hours.
When the beef has finished cooking, spoon some of the broth out of the pot into a bowl. Whisk in the xanthan gum and pour the broth back into the pot and stir to coat. Let thicken for a few minutes. Enjoy!