Prep ingredients: wash and cut brussels sprouts into halves, slice chicken tenderloin into 1-2 inch pieces, and mince garlic.
Heat a large cast iron skillet or wok, then add ~ 1 tbsp vegetable oil (more if needed). Add Brussels sprouts and cook until they are browned. Stirring continuously. If the skillet looks dry throughout the process (no more oil on the bottom), add more vegetable oil.
Once Brussels sprouts are brown on the outside and soft on the inside (do not overcook), remove from skillet and set aside.
Add more vegetable oil if needed. Then add garlic and chicken. Cook chicken, stiring continuously, until it is very close to being done, then add Brussels sprouts back into the skillet.
Add all three ingredients under "sauce" to the skillet. Stir to combine.
Once chicken is cooked, remove everything promtly.
Serve and enjoy! Optional: add more Thai Chili Sauce to the dish when serving.