First, fill a pot big enough to cover the ham with water. Simmer the ham for 30 minutes to de-salt it.
Mix the next 8 ingredients in a sauce pan and simmer 1 minute. In a large baking dish or roaster, scatter roughly sliced mushrooms and onions on the bottom of the baking dish or roaster. Drain the water off the ham and place it on the vegetables. Pour the barbeque sauce over the ham and cover it tightly with foil.Bake the ham at 250 degrees for about four hours, basting every hour.