Preparation / Directions:
1. Preheat oven to 425ø.
2. In a small heavy skillet cook leeks and prosciutto in oil over moderate heat, stirring, until leek is softened. Add water and cook, covered, over low heat until leek is soft, about 10 minutes. Pour off any excess water. Remove skillet from heat and add salt and pepper to taste. Let filling cool completely and stir in cheese.
3. Roll out puff pastry on lightly floured surface into a 12" square and with a 4" round cutter cut out 4 rounds.
4. Divide filling among rounds, mounding it in center, and brush edges of rounds with water. Fold rounds in half, pressing edges together firmly, and crimp edges with tines of fork to seal well. (Empanadas may be prepared up to this point 1 day in advance and kept covered and chilled.)
5. Mix egg with water to form egg wash. Arrange empanadas on a dampened baking sheet and brush with egg wash. Prick top of each empanada several times with fork (for steam vents). Bake empanadas in upper third of oven until puffed and golden, 12 to 15 minutes.