Add all the ingredients for the sauce into a screw-top jar and shake to combine. Taste and adjust seasoning. Add to a small saucepan and cook it over a medium heat for 4-6 minutes until it reduces slightly and becomes syrupy. Taste again and balance the flavours. Set aside.
Remove the dry outer leaves if any, from the bok choy. Halve it lengthwise and remove and discard the hard white core at the bottom. Pull each leaf apart with your hands and rinse well under running water to remove any dirt from them.
Chop the bok choy into small pieces (I chopped mine quite fine, but you could leave it more chunky as well.)
Heat a pan and add in the thicker light-green and white parts first. Toss for about 1-2 minutes until they start to wilt slightly.
Tip in the leafy top ends and turn the heat off. Pour the sauce and toss it all together. Garnish with sesame seeds and serve immediately.