Preheat oven to 200˚C.
Roughly chop the leek. Melt the butter in a pan and add the leek and lemon juice, sauté until soft.
Add the crab and herbs, stir to combine and remove from the heat.
Lightly flour your work surface and unroll the pastry. Cut 2 circles from the pastry roughly 16cm/6 inches in diameter, you could use a side plate as a guide to draw round. Spoon the crab and leek filling on to the pastry circles, laying the filling on one half of the pastry and leaving 1cm clear all around the edge. Brush the edges of the pastry with the egg then fold over to seal the pasty.
Place on a flat baking tray lined with parchment and bake in the oven for 30-35 minutes or until golden brown.