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1
One day before roasting: In a plastic container large enough to hold the ham, stir the water with the salt and brown sugar until dissolved.
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2
Add the spices.
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3
Score ham in a diamond pattern through the skin and fat, taking care not to cut into the meat.
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4
Add ham to brine, weight it with a plate to keep it submerged, and refrigerate for at least 4 hours but no more than 8 hours.
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5
Drain, rinse, pat the ham dry, and refrigerate.
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6
One hour before roasting, remove ham from the refrigerator.
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7
Preheat the oven to 450 degrees F.
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8
For the rub: Pulse the olive oil, mustard, parsley, sage, garlic, red pepper, salt, and black pepper in a food processor to make a paste.
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9
Rub it all over ham.
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10
In a large roasting pan, toss the onion wedges with 1 cup of the apple cider and set the ham on top.
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11
Roast the ham for 30 minutes, reduce the oven temperature to 325 degrees F, and roast until an instant-read thermometer inserted into the thickest part registers 165 degrees F, about 4 hours.
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12
After the first hour, loosely wrap aluminum foil around the bone to keep it from burning.
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13
Meanwhile, for the glaze: Boil, then simmer, the remaining apple cider in a saucepan, skimming as needed, until syrupy and reduced to about 2 cups, about 1 1/2 hours.
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14
During the last 1 1/2 hours of roasting the ham, brush it with the glaze every 30 minutes.
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15
Transfer the cooked ham to an ovenproof platter and let it rest in the turned-off oven for 30 minutes.
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16
Loosely cover the onions in an ovenproof bowl and put them in the oven as well.
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17
Strain the pan juices into a saucepan, skim off any excess fat, and bring to a boil.
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18
Make a paste with the flour and butter and whisk a bit at a time into the juices.
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19
Boil until thick.
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20
Carve the ham and serve with the onions and sauce.
