In a skillet heat the oil and add the peppers and onions. Cover and cook over low heat until tender but not brown. Blend the cornstarch with a little bit of chicken stock. Add the remaining stock and the soy sauce. Gently stir the shredded chicken and the stock mixture into the vegetables. Cook, stirring constantly, until the sauce is clear and thickened. Add the tomatoes and heat through. Serve over hot rice.
