Set the Anova Sous Vide Precision Cooker to 158ºF (70ºC).
Season the pork belly with salt and pepper. Place in a large zipper lock or vacuum seal bag with the olive oil and garlic. Seal the bag using the water immersion technique or a vacuum sealer on the moist setting.
Place the bag in the water bath and set the timer for 16 hours. Cover the water bath with plastic wrap to minimize water evaporation.
About 30 minutes before the pork is finished, prepare the salad: Combine the fish sauce, lime juice, palm sugar, and chiles in a large bowl. Mix well.
Add the mango, basil, cilantro, and peanuts to the fish sauce mixture. Mix well and let rest until pork is finished.
About 5 minutes before the pork is finished, heat the broiler to high.
When the timer goes off, remove the bag from the water bath. Remove the pork from the bag and pat dry with paper towels. Discard the cooking liquid.
Transfer the pork to a foil-lined broiler-safe baking dish. Generously brush pork with char siu sauce.
Broil pork for 5 minutes, and then brush a second layer of char siu sauce over the pork. Continue to broil until sauce is caramelized, about 5 more minutes.
Transfer pork to a serving plate and let rest for 5 minutes before serving with the mango salad.
