Briefly blanch the mung bean sprouts in boiling water (15 - 30 seconds). Drain thoroughly.
Cut the chicken into cubes that are between 1/2 and 1 inch (it's easiest to do this if the chicken is partially frozen). Add the marinade ingredients. Marinate the chicken for 15 minutes.
While the chicken is marinating, prepare the sauce and remaining vegetables. In a small bowl, combine the hoisin sauce, dark soy sauce, red wine vinegar, sugar, and water or chicken broth.
In a small bowl, combine the 2 teaspoons cornstarch with the 4 teaspoons water and set aside.
Rinse the green onions thoroughly and chop on the diagonal into 1-inch pieces. Peel and chop the onion and garlic.
Heat a wok on medium-high to high heat. Heat 2 tablespoons oil, swirling so that it coats the sides.
When the oil is hot, add the chicken. Stir-fry until it changes color is approximately 80 percent cooked. Remove the chicken. Clean out the wok.
Heat 1 tablespoon oil in the wok. When the oil is hot, add the garlic, green onion, and hot pepper flakes. Stir-fry for 15 seconds, then add the chicken back into the pan. Stir-fry briefly, then push to the sides. Add the sauce in the middle. Give the cornstarch/water mixture a quick stir and add to the sauce, stirring. Continue stirring until the sauce thickens.
Add the chopped onion, cook for a few more minutes. Stir in the bean sprouts, cook for another minute and serve hot.