Place the rings of onions in your slow cooker. Lightly rub the pork shoulder with small amount kosher salt and pepper and place in slow cooker. Brush on a generous layer of char siu sauce, coating evenly. Turn the slow cooker on low and leave on overnight or high, roughly 4-5 hours.
In large bowl, combine the cabbages, rau rau, mint, basil and toss well. Season with nuoc mam cham to taste. For best results, allow the slaw to marinate for at least 30 minutes before serving.
The slaw can also be made a day ahead of time. The longer the slaw marinates, the more liquid it releases--just drain this before serving.
When the pork is done, it will easily fall apart with gentle pressure--remove the pork into a large mixing bowl and pull using tongs or forks. The pork will release a lot of fatty juices which you can discard--but we like to leave keep a small amount to mix in the pulled pork.
Now add char siu sauce, 1-2 tbs at a time and coat the pulled pork well. Don't over do it--we like to keep extra char siu sauce on the side those who want more.
Add the pulled pork to the toasted baguettes and drizzle additional char siu sauce if desired.
Generously top with slaw and garnish with fried shallots.