Cut the chicken into 1 inch strips and stir-fry it in the oil over high heat for about 2 minutes, until it has browned and is barely cooked through. Drain the chicken and set it aside.
Steam the broccoli flowerets and the sliced stems for 3 minutes, and the snow peas for 1 minute. Set them aside.
Boil the chicken stock, ginger, and garlic for 5 minutes and then strain the stock.
Combine the cornstarch with the cold water. Stir it into the chicken stock and boil the mixture until it has thickened. Add the soy sauce, sherry, and sesame oil. Add the chicken and vegetables to the sauce and heat through. Serve with boiled or steamed rice.