Roll out the dough, trying to have it as round as possible on a flour-dusted surface. Take it to 2mm thick. Any thinner than that at it will break apart on handling, and if it's thicker, it would be too doughy.
Transfer the dough onto the baking tray you'll use to cook it on. This is important! You won't be able to move it once the filling is in place.
Break apart the ricotta and sprinkle it on the dough. Keep 4cm / 1.5in around the edge without covering.
Lay the leek and mushrooms on top of the ricotta and move them around so they're flat. Remember to keep that edge clear of filling.
Fold the edges towards the inside and paint them with the beaten egg and water.
Cook in a preheated oven at 180°C / 350°F for 40min or until golden.