Use 1 Tbsp of salt and sprinkle over the carrot and radish. Rub the salt in and let it stand for about 15 minutes. Use your clean hands and squeeze the extra liquid out from the radish and carrot (you’ll be surprised by how much the salt extracted the water out). Rinse with cold water and drain off. Place the vegetables in a glass jar or container. Add in sugar and vinegar. Stir to mix everything and store in refrigerator and let it marinate overnight
Assembling the sandwich:
Spread both cut sides of the bread lightly with butter on one side and mayonnaise on the other. Lay few pieces of char siu slices on it. Top with carrot and daikon shreds and a few pieces of minced chiles, if using, and coriander, spreading them along the length of the sandwich. Close and press firmly but gently to make the sandwich hold together. Serve whole or cut crosswise in half
Notes
Recipe for the char siu is here: http://whattocooktoday.com/char-siu-chinese-barbecued-pork.html