1 large or 2 medium leeks, slit lengthwise, rinsed, then thinly sliced crosswise, white and pale-green parts only (about 1 cup)
1 cup low-sodium soy sauce
6 Tbs. Shaoxing or dry sherry
1/4 cup peeled, julienned fresh ginger (cut into 1/8 x 1/8-inch matchsticks; from a 3-inch piece)
2 Tbs. granulated sugar
1 Tbs. slivered garlic (3 medium cloves)
2 cinnamon sticks
1 star anise pod
4 lb. boneless pork shoulder (also called butt), trimmed and cut into 4 large pieces
2 cups jarred roasted peeled chestnuts
2 Tbs. plain rice vinegar, more to taste
Cooked medium-grain white rice or udon noodles, for serving
Thinly sliced scallions, for garnish
Stir the leeks, soy sauce, Shaoxing or sherry, ginger, sugar, garlic, cinnamon sticks, and star anise pod in a 6-quart slow cooker. Nestle the chunks of pork shoulder into the mixture. Sprinkle the chestnuts on top.
Cover and cook on low until the pork is fork-tender 5 to 7 hours. Discard the cinnamon sticks and star anise pod and stir in the rice vinegar.
Break the pork into smaller chunks and serve on the rice or noodles with the chestnuts, a little of the sauce, and a sprinkle of scallions.