Season the pork with salt.
This step and the next are optional: Put a flameproof slow cooker insert on the stovetop over medium-high heat, then add the oil and warm until hot. (Or, use a frying pan for this step). Working in batches if necessary to avoid crowding, add the pork pieces and sear them on the first side without moving them until well browned, 3 to 4 minutes. Turn the pieces and sear on the second side until well browned, 3 to 4 minutes longer. Transfer to a plate and set aside.
Pour off all but 2 tablespoons of the fat from the insert and return the insert to medium-high heat. Add the yellow onion and sear, without stirring, until browned, about 5 minutes. Stir in the garlic, ginger, and 1 cup of the broth and deglaze the insert, stirring and scraping up any browned bits from the insert bottom, then let simmer for 1 minute.
If you have included the browning step, transfer the insert to the slow cooker, add the leek, mushrooms, and the remaining 7 cups of broth, and stir to combine. If you are starting here, combine the pork, yellow onion, garlic, ginger, leek, mushrooms, and broth in the slow cooker. Cover and cook on the low-heat setting for 8 hours. The pork should be very tender and the broth should be fragrant.