To make the pastry, combine the flour, cheese and salt in a bowl. Add the butter, and rub into the flour using your fingertips until it resembles coarse breadcrumbs. Drizzle in enough iced water, a tablespoon at a time, to bring the dough together using your hands. Divide the dough in two and flatten into squares. Cover with plastic wrap and refrigerate for at least 30 minutes.
Preheat the oven to 180°C. Line a baking tray with baking paper.
Heat the olive oil in a frying pan over a low heat. Add the leek, garlic and a pinch of salt and sauté for about 10 minutes until soft and translucent. Set aside in a large bowl.
Place the damp greens in a large saucepan and cook for 3–4 minutes over a low heat, stirring occasionally, until just wilted. Set aside to cool, then squeeze any moisture from the greens. Roughly chop and add to the bowl with the leek.
Combine the ricotta, cream, most of the egg (leaving a little to make an egg wash for the pastry), lemon zest and nutmeg in a small bowl and mix until smooth. Add to the greens and leek mixture and stir to combine well. Season to taste and set aside.
Remove the dough from the fridge and allow to rest at room temperature for 10 minutes. Roll each piece into a 25 cm square. Place one piece of pastry on the prepared baking tray and arrange the filling on top, leaving a 1 cm border of pastry all the way around. Whisk 1 teaspoon of water into the remaining egg to make a wash, and brush around the border of pastry. Top with the second piece of dough, trim any excess, then pinch the edges together to seal, marking with your index finger to create a crimped effect.
Brush the top of the pie with the remaining egg wash and poke a hole in the centre of the pie using a small, sharp knife to allow steam to escape during cooking. Bake for 45–50 minutes until golden and cooked through. Serve warm or at room temperature.