Trim the thick stems from the Vietnamese Mustard Greens (cải bẹ xanh) and separate the leaves; rinse them well in a bowl of Rice-Rinsing Water to loosen dirt, then give a final rinse under running Water and drain thoroughly.
Cut the greens into 3–4 inch pieces (or leave whole if preferred) and place them in a large mixing bowl.
Sprinkle a generous amount of Non-iodized Salt over the greens and massage/squeeze them for 5–10 minutes until they begin to soften and release liquid; this draws out moisture and helps preserve the texture.
Pack the salted greens tightly into a clean glass jar or fermentation vessel, pressing down so the liquid released covers the vegetables as much as possible.
In a small saucepan combine about 2 cups of Water with Sugar and a couple of teaspoons of Non-iodized Salt; heat just until the sugar and salt dissolve, then remove from heat and let the brine cool completely.
Stir a few tablespoons of the reserved Rice-Rinsing Water into the cooled brine (the rice water helps introduce natural yeasts and a pleasant complexity), then pour the brine over the packed greens so everything is submerged.
Tuck chopped Scallions and sliced Red Thai Chilies into the jar among the greens for flavor and aroma, and weigh the vegetables down with a clean weight or small plate to keep them submerged under the brine.
Cover the jar loosely (or use an airlock if you have one) and leave at cool room temperature for 2–5 days to ferment, checking daily to press the greens down and taste; when the sourness and flavor are to your liking, seal and refrigerate to slow fermentation.
Serve the dưa chua chilled or at room temperature as a crunchy, tangy condiment alongside rice, porridge, noodle soups, or grilled meats.
