Recipe adapted from "Food From Many Greek Kitchens," by Tessa Kiros. Copyright 2011 by Tessa Kiros. Used with permission of Andrews McMeel Publishing.
Combine lemon juice, oil, oregano, and water in a large nonreactive baking dish. Rub lamb all over with salt and pepper, and place in marinade, turning to coat well. Cover with plastic wrap, and refrigerate 24 hours, turning once or twice.
Preheat oven to 350 degrees. Transfer lamb and marinade to a roasting pan. Cover lamb with parchment, and cover pan tightly with heavy foil. Roast 1 hour, then flip lamb. Cover again, and reduce heat to 300 degrees. Roast lamb 2 hours.
Flip lamb gently, since it will be very tender. Add potatoes to pan, and sprinkle with a little salt. Cover, and roast 1 hour. Raise heat to 400 degrees. Uncover, and roast until potatoes are golden around edges, 15 to 20 minutes.