Mix the sauce ingredients together in a small saucepan, then gently simmer until hot. Heat a griddle pan until very hot. Rub the five-spice all over the steak, then season with a little salt and pepper. Sear steak for 2-3 mins on each side until cooked to your liking, then leave to rest.
Meanwhile, gently simmer or steam the bok or pak choi for 4-5 mins until wilted, but still crunchy. Toss the greens in a few tbsps of sauce and the sesame oil. Cut steak into thick slices. Serve on a plate with a small pot of sauce, the greens and some steamed rice.