Pat the chicken fillets dry with paper towels. Cut each thigh into six to eight strips. Place in a bowl.
Add the minced garlic, sliced chilies, salt, pepper and sugar to the chicken. Mix well. Set aside.
Heat the cooking oil in a wok or frying pan. Add the chicken and stir fry for five to seven minutes, depending on the size of the strips into which you have cut the fillets (see stir frying basics).
Pour in the balsamic vinegar. Stir fry for half a minute.
Add the sliced onion and diced bell pepper. Stir fry for one minute.
Add the sliced mangoes to the chicken and vegetables. Stir fry just until the mango slices are heated through.
Serve the spicy stir fried chicken and fresh mangoes immediately.