In a small bowl, combine salt and brown sugar. Rub mixture all over the pork, cover and refrigerate overnight.
Preheat oven to 300 degree F.
Wipe off the salt-sugar mixture from the pork with paper towels. Place the pork in a roasting pan. Set aside.
In a bowl, combine together maple syrup, lemon juice, mustard and freshly ground pepper. Reserve ½ cup of the mixture for dipping sauce.
Baste about ½ of the remaining maple-mustard mixture all over the pork. Roast in the oven for 6 hours or until meat is fork tender, basting the pork with the rest of the maple-mustard in the last hour of cooking.
Make the dipping sauce:
Heat oil in a sauce pan over medium heat. Add onion and cook until translucent. Stir in ½ cup reserved maple-mustard sauce. Reduce heat and let simmer for a minute. Add butter and whisk until sauce is silky smooth. Season with freshly ground pepper.
Tear and shred the pork into chunks and place on a serving platter. Serve as is with the dipping sauce or as a sandwich.