In a small bowl, whisk the chicken stock with the fish sauce and cornstarch. In a wok or a large skillet, heat the oil until very hot. Add the chicken and stir-fry over high heat until opaque but not cooked through, about 2 minutes. Using a slotted spoon, transfer the chicken to a plate. Add the leeks, ginger and chile to the wok and stir-fry over high heat until slightly wilted and lightly browned in spots, about 2 minutes. Return the chicken and any accumulated juices to the wok and stir-fry for 1 minute. Whisk the stock mixture and add it to the wok. Stir-fry until the chicken is just cooked through and the sauce is slightly thickened, about 1 minute. Serve with steamed rice.