1. Rinse ham with cool water; dry with paper towels. Let stand, uncovered, at room temperature 1 hour. Meanwhile, whisk together honey, orange juice, balsamic vinegar, and mustard; set aside.

2. Preheat oven to 300 degrees, with rack in lower third. Fit a roasting pan with a rack, and place ham, with the thicker rind on top, on rack. Scatter onion and orange wedges and rosemary around ham on rack. Transfer to oven, and cook 1 hour.

3. Remove pan from oven, and let ham cool slightly. Increase oven temperature to 350 degrees. Trim fat all over the ham to a layer of about 1/4 inch (it does not need to be perfectly even; the bottom will have less fat and more skin). Turn ham, bottom side down. Score fat on top of ham in a diamond pattern, each 1 to 2 inches, cutting about 1/4 to 1/2 inch through the fat and into meat. Baste with honey mixture. Add enough water to roasting pan to fill the bottom by about 1/4 inch.

4. Return ham to oven, and cook 1 hour more, basting often with remaining marinade (do not baste with pan juices). If necessary, add water to pan to keep juices from burning. Remove from oven; transfer ham to a serving platter; discard orange, onion, and rosemary. Let stand 30 minutes before carving ham.

5. Meanwhile, make gravy: Strain liquid from roasting pan into a liquid measuring cup or bowl, and skim off fat from surface with a large spoon. Place roasting pan over medium-high heat. Add cider vinegar, and simmer until most of the liquid has evaporated. Return defatted juices to pan along with 2 cups stock. Bring to a boil, and let simmer.

6. In a small bowl, whisk together remaining 1 1/2 cups stock and the flour; whisk into sauce. Continue simmering until liquid is reduced by half and slightly thickened. Season with salt and pepper. Serve hot with ham.