1. Brine the turkey: In a pot, combine brown sugar and salt with 3 quarts water. Boil until sugar and salt have dissolved. Remove from heat and stir in peppercorns, thyme, bay leaves, cloves, and garlic. Set aside to cool. Once cooled, pour over turkey breast and brine for 48 hours.
2. Make the rub: Preheat oven to 300 degrees. In a small bowl, combine juniper, coriander, and black peppercorns. Remove turkey from brine and press rub onto turkey. Roast until turkey reaches an internal temperature of 165 degrees, about 2 hours. Let rest for at least 15 minutes, then slice ¼-inch thick.
3. Assemble the sandwich: Place two slices of turkey pastrami on a baking sheet and top with slices of Munster cheese. Heat under oven broiler until cheese is melted. Meanwhile, toast rye bread. Pile turkey pastrami onto rye bread, then top with melted cheese slice. Add sauerkraut and spicy mayo to taste and top with remaining rye slices.