Combine the flour, salt and egg in a large mixing bowl. Add 2 cups water and beat with a whisk for 5 minutes or until very smooth. Quickly stir in remaining water. Allow to stand 1 hour. (The batter will appear very thin, almost milk-like, but do not worry).
Stir together 1/4 cup lardo, prosciutto, rosemary, garlic and black pepper in a mixing bowl until smooth and set aside.
To cook borlenghe, heat a 14to 16-inch non-stick pan over medium heat and brush with a 1/4 teaspoon of remaining lardo. Pour enough batter to coat pan (just like a crespella) and return to heat. Cook 4 minutes on the first side and turn carefully. (The texture will not be like a crespella, or anything else, for this is an original and distinct delicacy. It may crack, but keep going). Continue until all the batter is finished, or about 10 to 12 borlenghe.
To serve, reheat borlenghe quickly in same pan, spread with 1 tablespoon prosciutto mixture, sprinkle with grated cheese and serve warm, folded into quarters.