Preheat the oven to 400°F Mix together the garlic, chili flakes, soy sauce, sesame oil and vinegar. Pour over the pork and leave to marinate for 5 minutes (or up to 30 minutes if you have time).
Heat the sunflower oil in a flameproof casserole pan, then brown the pork on all sides. Add the marinade, cover and cook in the oven for 25 minutes or until cooked through. Remove the pork from the pan, then leave to rest (covered) for 5 minutes. Pour off the juices and reserve.
Add the bok choy to the pan cut side down, brown over a high heat, cover the pan and cook gently for 5 minutes until just tender. Slice the pork thinly, pour over the hot juices and serve with the bok choy and plain steamed rice.