Preheat oven to 300 degrees F. Season pork with salt. Heat oil in a 4-quart Dutch oven over medium-high heat. Add pork in batches and sear until browned all over, 6 to 8 minutes. Transfer to a bowl. Add parsnips and onion to pan and cook for 4 minutes; remove to bowl with pork. Add sage, ginger, and garlic to pan and cook for 1 minute. Add 3 of the apples, broth, cider, and vinegar; bring to boiling. Reduce heat; cover and simmer 5 minutes or apples are tender. Puree mixture with an immersion blender or transfer, in batches, to a blender and blend until smooth. Add pork, parsnips, and onion. Bring to boiling.
Cover and bake for 2 hours or until pork is fork-tender. Stir in remaining apples and brandy, if desired. Serve topped with sour cream, additional sage, nutmeg, and/or pepper, if desired.