Preheat the oven to 325°F. Remove the rind from the ham. Leave a layer of fat to protect the meat. Use a paring knife to score a crosshatch pattern over the top and sides of the ham. Set aside.
Transfer several handfuls of hay to a large bowl and soak with water. Line the bottom of a dutch oven with a layer of hay, arranging some of the around the sides of the pot. Scatter the bay leaves and whole spices over the hay in the Dutch oven.
Place the ham in the Dutch oven on top of the hay. Stuff the remaining hay around the ham so that it is fully enclosed. Pour 1 cup of mead and the water into the Dutch oven. Place a small piece of parchment on top of the ham and then tightly cover the whole pot with foil. Cover with a lid and bake for 2 ½ to 3 hours or until it reaches an internal temperature of at least 145°F.
While the ham is baking, prepare the glaze. Add the remaining 1 cup mead to a small saucepan and bring to a boil over high heat. Boil the mead until reduces to a 1/4 cup. Turn the heat down to low and add the brown sugar, butter, grainy mustard, apple cider vinegar, and kosher salt. Whisk until the butter is fully emulsified. Set aside and allow to cool.
When the ham is fully cooked, remove from the oven and turn the heat up to 425°F. Transfer the ham from the Dutch oven to a roasting pan. Carefully scrape off as much of the hay as possible. The hay pieces can be sharp and unpleasant to eat.
Brush the ham with the glaze and roast for 15 to 20 minutes. Apply more glaze every 5 minutes until the ham is caramelized and well browned. Carve the ham into thin slices and transfer to a serving platter. Enjoy!