Preheat the oven to 275°. In a medium bowl, whisk the mustard with the brown sugar, salt, pepper, paprika and onion powder. Set the pork shoulder, fat side up, in doubled 14-by-18-inch disposable aluminum roasting pans. Brush the pork with the mustard mixture. Roast, uncovered, for 12 hours, until the meat is very tender and is pulling away from the shoulder bone.