Place pinto beans in a large bowl. Cover with cold water 3 inches above beans; cover and let soak 12 hours. Drain beans, and transfer to a Dutch oven. Add apple cider; bring to a boil over medium heat. Gently boil, uncovered and stirring occasionally, 30 minutes. Remove from heat; drain, reserving liquid. Layer 4 oz. thinly sliced salt pork in a 2-qt. Dutch oven. Spoon beans over salt pork; bury onions in beans. Cook dry mustard, sorghum syrup, and salt over medium heat, stirring often, 3 minutes or until mustard and salt dissolve. Pour mixture over beans; top with 4 oz. thinly sliced salt pork. Add reserved bean liquid and, if necessary, hot water to cover. Bake, covered, at 300° for 3 hours, adding hot water as needed. Bake 2 more hours or until beans are tender.
Adapted from Cider Beans, Wild Greens, and Dandelion Jelly: Recipes from Southern Appalachia by Joan Aller.