Preheat the oven to 375°. In a small bowl, combine the maple syrup, mustard and brown sugar. Using a sharp knife, remove the rind from the ham, leaving a layer of fat at least 1/4 inch thick all over. Score the fat in a crosshatch pattern at 1/2-inch intervals. Set the ham in a large roasting pan. Brush the ham with two tablespoons of the maple mustard glaze, and scatter the apple and onion slices around it. Add the cider and chicken broth to the pan. Cover the ham with foil and braise in the oven for 2 hours. Uncover the ham, and brush it with all but 2 tablespoons of the maple-mustard glaze. Bake uncovered about 1 1/2 hours longer, until the ham is deeply browned and an instant-read thermometer inserted in the thickest part of the ham registers 155°.