Step 1

Cut white and pale-green parts of leeks crosswise into 1/2-inch-thick slices. In a bowl of cold water wash leeks well and lift from water into a colander to drain. Pat leeks dry. In a large skillet cook leeks in butter over moderate heat, stirring, until tender, about 12 minutes, and cool. Cut salmon into roughly 3/4-inch pieces and in a bowl toss with leeks, dill, zest, salt, and pepper until combined well.

Step 2

In a small bowl whisk together egg and water to make an egg wash. On a lightly floured surface with a lightly floured rolling pin roll 1 puff pastry sheet into a 10-inch square and the other into a 12-inch square. Transfer 10-inch pastry square to a floured large baking sheet and mound salmon filling in center, forming a round 8 inches in diameter. Brush edges of pastry evenly with some egg wash. Carefully drape remaining pastry square over salmon and gently press edges together to seal. With a sharp knife trim edges of pastry to form a 10-inch round. Crimp edges and cut 4 steam vents on top of crust. Brush crust evenly with some remaining egg wash. Chill pie, loosely covered, at least 1 hour and up to 3.

Step 3

Preheat oven to 400°F.

Step 4

Bake pie in middle of oven until pastry is golden brown, about 30 minutes.

Step 5

Serve pie warm or at room temperature with sauce.