1. Rub the pork all over with the five-spice powder and salt. Add the chicken broth, soy sauce, brown sugar, sesame oil, and red pepper to the slow cooker. Stir to dissolve the sugar. Add the scallions, garlic, ginger, the mushrooms, if using, and the meat, turning it a few times to coat. Cover the cooker, set it on HIGH, and cook for 4 hours.
2. Set the cooker on LOW and cook until the meat is very tender, at least another 2 hours (6 hours total).
3. Transfer the pork to a platter, cover lightly, and let rest 15 minutes.
4. Meanwhile, pour the cooking liquid into a measuring cup and skim off and discard the vegetables and fat that rise to the surface. Slice the meat and serve warm or at room temperature with noodles and the sauce on the side.