2. Mix the salt, garlic, onion, and five-spice powders, and pepper. Sprinkle all over the pork chops. Place on a plate, cover, and let stand at room temperature while building the fire.
3. Build a fire in an outdoor grill. For a charcoal grill, let the coals burn until they are covered with white ash. Spread out the mound of coals into a bank, with one side about two coals deep, and the other side of the slope with a scattering of single coals. For a gas grill, preheat the grill on High. Leave one side on High and turn the other side to Low. In both cases, you will have two areas for cooking, one hot and the other cooler.
4. Lightly oil the cooking grate. Place the pork chops over the hotter area of the grill and cover the grill. Grill until the undersides are seared with grill marks, about 3 minutes. Turn, cover, and sear the other sides, about 3 minutes more. Move the chops to the cooler area of the grill. Grill, covered, until the pork chops feel firm when pressed in the centers with a finger, 12 to 15 minutes. Transfer to a platter and serve with the plum sauce.