Place the soy sauce, stock and cornstarch in a small bowl and whisk to combine. Heat the oil in a wok or large skillet set over medium-high. Add the bell peppers and onion and cook, stirring, for 2 minutes. Add the chestnuts, corn, garlic and ginger and cook 2 to 3 minutes more. Stir the soy sauce mixture again and then pour over the vegetables. Cook 1 to 2 minutes more, or until a thickened sauce forms around the vegetables. Serve immediately.