Add tamari, brown sugar substitute, and sherry; simmer for 10 minutes. Meanwhile, heat remaining sesame oil in a large pan, stir-fry onions for 2 minutes, then add chicken and cook until done.
Make a slurry with xanthan gum and water; add to sauce, whisk until thick. Pour sauce over chicken and coat evenly. Serve with lettuce wraps or cauliflower rice, garnished with sesame seeds.
Notes
Adjust cooking time as needed to ensure chicken is fully cooked.