To prep the choy sum, separate the leaves from the tougher stem parts. Give both a good wash.
In a bowl, mix together the Lao Gan Ma, mustard, honey, sesame oil, and soy sauce. Give it a quick whisk to bind the ingredients together. Then, add the water, bit by bit, to loosen the sauce until it gets to the consistency of a spoon-coating salad dressing.
Steam the choy sum until just tender. (The leaves usually take 5 minutes, the thicker, tougher stems 6-8 minutes.) When they’re done, toss the leaves and stem separately through the sauce, and serve warm!