Preheat oven to 400°F
Cut white and pale-green parts of leeks crosswise into 3/4” 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 skillet, cook leeks in butter over moderate heat stirring until tender, about 12 minutes. Transfer to a bowl, add lemon zest and dill. Combine and allow to cool for about 10 minutes.
Cut salmon into roughly 3/4-inch pieces and combine with leeks and set aside.
In a small bowl whisk together egg and water to make an egg wash.
On a lightly floured surface, cut puff pastry in two.
Roll out one of pastry sheets until it is about 12" wide and transfer on parchment paper in a standard 13” wide baking tray. Put filling at center of sheet and form into a mound about 10 inches in diameter. (Make sure you have at least a 1” border of pastry at the top and bottom of the pastry sheet).
Roll out the other pastry sheet until it is also about 12" wide and carefully drape and stretch It over the filling (Again make sure that you have at least a 1” border of pastry at the top and bottom of the pastry sheet).Gently press edges of both sheets to seal the filling. With a sharp knife (or scissors), trim edges of both pastry sheets allowing and inch wide strip of pastry all around.
Crimp edges and cut 4 steam vents on top of crust.
Chill pie in refridgerator for half an hour.
Brush chilled pie with egg wash.
Bake pie in middle of oven until pastry is golden brown, about 30 minutes.
Serve pie warm or at room temperature.