Remove ham from the original packaging and throw away the glaze packet and the plastic disk that covers the bone. Place the ham in the large oven bag and tie closed. Cut four slits (about 2 inches each) in the top of the bag. Transfer the bagged ham to a roasting pan and let it sit at room temperature for about 1 1/2 hours.
Heat the oven to 300 degrees and bake the ham for 1 1/2 to 2 1/2 hours (until the internal temperature of the ham is about 110 degrees). About an hour into baking, start making the glaze by combining the jelly, 1/2 cup brown sugar, maple syrup, whole-grain mustard, cinnamon and nutmeg in a medium saucepan. Bring the mixture to a boil and reduce the heat to low, simmering, until the mixture is very thick and syrupy, about 45 minutes. Stir often while it simmers (it should reduce to about 1/3 cup). Off the heat, whisk in the butter.
Combine the remaining brown sugar and dry mustard together in a small bowl. Remove the ham from the oven and let it sit for 5 minutes. While it rests, increase the oven temperature to 400 degrees. Carefully cut open the oven bag and let it fall to the bottom of the ham so the entire ham is exposed. Using a pastry brush, paint the glaze onto the top and sides of the ham. Carefully press the sugar mixture onto the outside of the ham with your hands. Return the ham to the oven and bake until it is dark brown and caramelized, 14-16 minutes.