Place the meat skin side up on a cutting board. With a sharp heavy knife, cut through the rind and fat to make a crosshatch pattern with 1-inch squares. With a thin sharp knife, make a small slit in each square. (This is where you will insert the cloves; you could insert them now, but I have found that all the handling is likely to break off most of the nice-looking tops of the cloves.) Rub the meat with the salt and pepper.
Place the meat in a baking dish. Cut 1 orange in half and squeeze the juice over the meat. Slice another orange and place the slices under and over the meat. Cover and refrigerate for 1 to 2 days, turning three or four times.
Preheat the oven to 400°F.
Discard the orange and place the meat fat side down in a roasting pan. Insert the sage between the cut ribs. Turn the meat skin side up. Cut the remaining orange in half and squeeze the juice over the meat. Cut the orange halves into smaller pieces and place them under the meat. Insert a clove in each of the slits in the fat.
Cover the roast with foil. Place the roasting pan in the middle of the oven and roast for 45 minutes. Remove from the oven and reduce the temperature to 350°F. Remove the foil. Remove the meat from the pan and add potatoes and onions to the pan, making a foundation for the meat. Return the meat to the pan and place in the oven, this time setting it in the lower third of the oven; roast for about 1 hour.
Move the pan to the middle of the oven. Increase the heat to 400° to 425°F and roast for about 20 minutes longer under frequent surveillance to produce a good crackling. If you have a top oven broiler, you could use it, but be very careful that you don’t burn the roast. It is difficult to give the exact time for the crackling process, since that depends on the intensity of heat in your oven. If there is no sign of crackling after 10 minutes, gradually turn the heat up to 450°F. The crackling will be superb if it looks as if the rind is boiling and the fatty squares have started to separate. Remove and allow to cool slightly.
Cut the meat into 2-rib sections and place on serving plates. The meat looks nice served with the cloves, but be advised that they should be removed before eating, as they are still quite strong-tasting.