Preheat the oven to 300 degrees . In a large, heavy ovenproof pot or Dutch oven, whisk the first 3 ingredients. Add the pork and onion and bring to a simmer over medium; cover. Roast until the pork is tender, 1 3/4 hours. Remove the pot from the oven and increase the heat to 400 degrees . Using 2 forks, shred the pork in the pot.
In a bowl, stir the creamed corn, egg and sour cream, then gently fold in the muffin mix and scallions. Spoon the batter to cover the pulled pork. Bake, uncovered, until cornbread is golden and tester comes out dry, 20 to 25 minutes.