Rinse the ham and pat dry. To score the ham, use a serrated knife to make parallel cuts about an inch apart, across and through the skin into the fat. Cut diagonally across these first cuts to make a diamond pattern.
In a food processor combine the parsley, sage, garlic, mustard, red pepper, salt, and pepper. Add 1/4 cup of the oil and pulse to a make a coarse paste. Rub the paste all over the ham, pressing it into the cuts. Cover the ham with plastic wrap, put on a pan, and refrigerate overnight.
Remove ham from the refrigerator about 1 1/2 hours before cooking so the meat can come to room temperature. Preheat the oven to 450 degrees F.
In a large roasting pan toss the onion wedges with the remaining oil. Place the ham on top of the onions and add 1 cup of the cider. Roast the meat for 30 minutes, reduce the oven temperature 300 degrees F. Cook the ham until an instant read thermometer inserted into the thickest part registers 150 degrees F. A 13-pound ham will take about 4 to 4 1/2 hours, about 20 minutes per pound.
Meanwhile in a large sauce pan bring the cider to a boil. Reduce the heat to a brisk simmer and cook the cider until syrupy and about 1 1/2 cups. Skim off any foam that comes to the surface as the cider reduces. During the last 1 1/2 hours of cooking the ham, brush the reduced cider over the roast every 30 minutes to glaze the skin.
Let the cooked ham rest, loosely covered with foil, at room temperature for 30 minutes before carving. Reserve the onions and pan juices, skimming off any excess fat. Carve the ham and serve with the onion and pan juices.