Cook pork, onions and celery well in large non-stick skillet at medium-high heat, breaking up with spoon for 8 minutes or until cooked through; drain off fat. Remove from heat.
Unroll one sheet of pastry on lightly floured work surface. Cut into 9 squares. Repeat with remaining sheets.
Mix egg with water. Spoon scant 1 tbsp (15 mL) filling onto centre of one square. Brush edges with egg mixture. Fold over into triangle; pinch seams and press with tines of fork to seal. Repeat with remaining squares and filling. Arrange 1 inch (2.5 cm) apart on large parchment-lined baking sheets. Pierce tops with paring knife to create steam vents.
Brush with egg. Bake for 18 minutes or until golden brown.