In medium bowl, whisk together the orange juice, soy sauce, brown sugar, hoisin sauce, rice wine, ginger, mirin, chili garlic sauce, fish sauce, cornstarch and sesame oil. Set aside.
Slice the chicken into bite size pieces. Season with salt and pepper. Set aside.
Place a high sided skillet or wok over medium heat. Add the olive oil and garlic. Cook the garlic for 1 minute or until fragrant. Remove the garlic from the pan and reserve.
Add the chicken to the pan and cook for 3-4 minutes or until brown on all sides and cooked through. Transfer the chicken to a plate and keep warm.
Add the onion to the pan, cook for 2 minutes. Stir in the bell pepper, cook for an additional 1-2 minutes,
Add the chicken to the vegetables. Stir to combine.
Pour the sauce over the beef stir fry and toss to combine. Cook for an additional 2-3 minutes tossing a few times to allow the sauce to thicken and caramelize.
To serve, top the stir fry with sliced green onions and toasted sesame seeds, if desired.
Notes
Mongolian Chicken can be made up to 3 days in advance. Store, refrigerated, in an airtight container. Reheat in a skillet over medium high heat or in the microwave until hot.