With the tip of a small, sharp knife, cut many slits in the skin of the lamb. Insert the garlic slivers into the slits. In a small bowl, combine the olive oil, lemon juice, oregano, salt, rosemary, and pepper. Brush the lamb generously with this mixture. Place the lamb in a shallow roasting pan. Cover and refrigerate for 24 hours. Cover the remaining basting mixture and set aside.
Uncover lamb and bring to room temperature. Preheat the oven to 325 degrees F. Roast the lamb for 1 hour. Drain off any fat that has accumulated in the bottom of the pan. Pour the hot water into the pan. Continue to roast the lamb, basting every 20 minutes, first with the remaining basting sauce, then with the juices that accumulate in the pan. Allow 20 minutes per pound for total roasting time, or until a meat thermometer inserted into the thickest part of the lamb, with out touching fat or bone, registers 140 degrees for rare, 160 degrees for medium, and 170 degrees for well done. Remove lamb from the oven and set aside, lightly covered for about 20 minutes before carving. Skim the fat from the juices in the pan and strain through a fine sieve.
In a skillet heat olive oil over moderate heat and saute shallots until tender. Add wine and simmer until reduced by half. Add the strained pan juices and chicken stock and bring to a simmer. Whisk in the half of the butter and flour mixture and simmer until thickened. If necessary whisk in remaining butter and flour mixture. Taste sauce and adjust seasoning with salt and pepper.