Grate oranges to yield 1 tablespoon zest; place zest in a medium bowl. Refrigerate zested oranges until ready to roast with ham. To bowl with zest, add salt, brown sugar, garlic, thyme, rosemary, and fennel seeds; stir to combine.

Place ham flat, cut-side down, on a cutting board. Using a sharp knife, remove skin, leaving a ¼-inch to ½-inch layer of fat intact. Cut fat in a diagonal crosshatch pattern at 1-inch intervals, taking care not to cut into the meat. Place the ham on its side. Cut 1 (4-inch-long) horizontal pocket about 2 ½ to 3 inches deep in the center of flat cut side, being careful not to poke through opposite side.

Rub half (about ½ cup) of the salt mixture into the ham pocket. Rub exterior of ham all over with remaining salt mixture (about ½ cup). Transfer ham to a large plate or a rimmed baking sheet, and refrigerate, uncovered, for at least 12 hours or up to 24 hours.

Adjust oven rack to lowest position and preheat oven to 325°F (160°C). Set a roasting rack in a large roasting pan. Slice reserved oranges into ⅓-inch-thick slices; place in an even layer on rack bottom. Place ham, flat-side down, centered on orange slices. Pour 3 cups water into roasting pan. Bake until a thermometer inserted into center of ham, close to but not touching the bone, registers 130°F (54°C), 2 ½ to 3 hours. Remove ham from oven, and increase oven temperature to 450°F (230°C).

Whisk together maple syrup, bourbon, mustard, and black pepper in a small bowl. Using a pastry brush, brush ham with half (about 2 ½ tablespoons) of glaze; bake until glaze is set and starting to brown, 10 minutes. Brush ham with remaining glaze; rotate pan, and continue baking until an instant-read thermometer inserted into center of ham registers 145°F (63°C), 8 to 10 minutes longer.

Transfer ham to a carving board, flat-side down, and let rest for at least 30 minutes or up to 2 hours. (As it rests, the ham's internal temperature will increase as it rests to about 150°F to 155°F.) Carve ham into ¼-inch-thick slices. Arrange slices on a platter, and moisten lightly with remaining pan juices. Serve, passing remaining pan juices separately.