Heat the oven to 325°F with a rack in the middle position. Set a wire rack in a rimmed baking sheet. In a small bowl, combine 2 tablespoons salt, 1 tablespoon black pepper, the sugar, ground fennel, pepper flakes, granulated garlic and 4 teaspoons of rosemary. Rub the mixture between your fingertips until well combined and the rosemary is fragrant. Sprinkle the mixture evenly over both sides of the racks of ribs, then rub it in. 

Place the ribs, meaty side up, on the wire rack on the baking sheet. Transfer to the oven, then pour 3 cups water into the baking sheet. Bake until a skewer inserted into the meat between the bones meets no resistance, 2½ to 3 hours. Transfer the rib racks to a cutting board and let rest for 20 minutes. Cut the ribs between the bones and transfer to a platter.