In a Pyrex dish or bowl, combine the soy sauce and garlic.
Place the chicken into the dish, cover and refrigerate for several hours or overnight to marinate.
Heat the oil in a wok or large sauté pan over high heat. Add the chicken, marinade and onion, and sauté quickly until chicken is cooked through. Add in broccoli and turn heat to medium-high. Cook for 3 to 4 minutes. Toss in snow peas and mangoes with their juices and cook for 2 to 3 minutes. Toss in the cilantro and scallions and serve.