Thinly slice the chicken breasts into bite-size pieces and season them with pepper. Meanwhile, chop the broccoli into bitesize florets, mince the garlic, grate the ginger, and chop the onion. Add 2 tbsp of oil into a large skillet over high heat.
Once the oil is hot, add half the chicken and spread it into one layer. For the first 3-4 minutes, don’t stir to allow the chicken to caramelize from the bottom. Stir and cook for another 2 minutes, then remove it and transfer it to a plate. Repeat with the second batch of chicken.
To a small bowl, add the broth, soy sauce, vinegar, sesame oil, erythritol, and xanthan gum. Whisk until the sweetener is dissolved. Set aside.
Add the remaining oil to the skillet. Add the onion and cook for 2-3 minutes. Then, add the broccoli, garlic, ginger, and pepper flakes.
Cook for another 1-2 minutes, constantly stirring, until fragrant. Pour the sauce over the broccoli, add the chicken, and stir. Bring the sauce to a boil, then reduce the heat to low and let it simmer for 2-3 minutes until it thickens. If the sauce is still thin, add ¼ or ½ tsp xanthan gum until you reach your desired consistency.
Take the pan off the heat, taste, and adjust seasoning if needed. Divide between 4 plates. Garnish with sesame seeds, and serve immediately.