Combine the sesame oil and xanthan gum in a medium saucepan and stir until the xanthan gum is fully mixed with the oil. Slowly whisk in the apple cider vinegar so that it mixes in well with the xanthan gum, then pour in the water, soy sauce, ketchup and sweetener.
Bring the sauce to a boil, reduce to a simmer and cook until thick, 1-2 minutes. Pour over your favourite meal or use for dipping!