Preheat oven to 350 degrees. With a large fork, make deep holes for the marinade to seep into. Rub the roast with mustard and sprinkle liberally with pepper. Put into a glass or ceramic baking dish. Combine lemon juice, bourbon, and salt. Pour juice over pork and cover with plastic wrap. Let marinate at least 4 hours in the refrigerator. turn pork several times to marinate. Bake at 350 degrees for 1 1/2 hours, basting occasionally. When the meat reaches 175 degrees ( using a thermometer), transfer to a carving board and cover with foil to keep warm. Scrape the meat drippings from the baking dish and transfer with juices and leftover marinade to a small saucepan. Skim off fat and add lemon slices and water and simmer until lemon is tender ( 10 minutes). Carve the meat and pour over sauce and lemon slices. Serves 6.