Preheat the oven to 450 degrees F (230 degrees C).

Rinse roast and pat dry with paper towels. Mix soup mix and pepper together in a small bowl; rub generously over all sides of roast. Place roast onto a clean, dry baking sheet; let rest for 30 to 45 minutes.

Mix rock salt, flour, and rosemary together in a bowl. Gradually pour in water and mix by hand (wear gloves if desired) until batter is thick and paste-like, adding more water if necessary.

Spread salt batter into the bottom of a roasting pan to make a 1/2- to 3/4-inch-thick layer; reserve 1 to 2 cups batter. Place roast, with the bones facing down, on batter in the center of the pan. Pull batter up and around roast until meat is completely covered. Pat on extra batter if necessary. Insert an oven-safe meat thermometer into the roast so its gauge will be visible from the front of the oven.

Cook in the preheated oven for 15 to 20 minutes. Remove from the oven and patch any holes or breaks in the coating with remaining salt batter.

Return roast to the oven and cook until internal temperature reaches 130 degrees F (54 degrees C), about 3 hours, or to desired doneness. Remove from the oven and let rest until internal temperature reaches 140 degrees F (60 degrees C), 20 to 30 minutes.

Chip off and discard salt crust, then slice and serve.