Rinse lamb shanks and pat dry.
In an 11- by 17-inch roasting pan, combine broth, sautéed onions and garlic, and sherry. Arrange lamb in a single layer in mixture. Cover pan tightly with foil.
Bake in a 400° oven until meat is very tender when pierced, 2 to 2 1/2 hours.
With tongs, transfer shanks to a platter. Skim and discard fat from pan juices. Stir in red peppers and vinegar. Let lamb and red pepper sauce cool to room temperature, about 40 minutes.
Put 1/2 the shanks in each of 2 foil pans (9 by 13 in.). Spoon sauce evenly over lamb. Seal pans with foil, and freeze (see Cold Facts below).
Thaw 1 pan (see notes). Bake, covered, in a 400° oven until lamb is steaming, about 1 hour. Uncover and bake until meat is well browned, about 10 minutes longer. Season to taste with salt and pepper.
Cold Facts. Packaging: freezer bags and foil pans. Squeeze out all air and seal bag. For better protection of frozen foods during storage, seal each bag inside a second one. Lay bags flat in freezer until solid so they will stack. Seal foil pans with a double layer of foil to protect foods for storage. Label each dish with permanent marker. Make a master list of the dishes. As you remove each from the freezer, cross it off the list. For best quality, freeze meat mixtures no longer than six weeks, others three months.
Note: Nutritional analysis is per serving.