Boil the rice in 130ml water with 1tsp of the salt for around 12-15 minutes until cooked, then set aside.
Preheat the oven to 180C/160C F/ Gas 4. Melt the butter in a large frying pan on a medium heat. Sprinkle the remaining 1tsp salt over the leeks, add them to the pan and sauté for 5 minutes. Add the rice and mint and cook for 5 minutes. You want the leeks to just soften while still retaining their crunch. Taste and adjust the seasoning, set aside to cool a little, then mix in the egg.
To assemble, line a large baking sheet with baking paper and place one puff pastry sheet on it. Brush the edges with egg and prick all over with a fork. Spoon the filling on top, leaving a narrow margin on all sides. Place the other pastry sheet on top and press the edges with the tines of a fork to seal together. Prick the pastry all over, brush with more egg to glaze, and sprinkle the sesame seeds over.
Bake for 15-20 minutes on a lower rack until golden brown. Remove from the oven and place a clean tea towel over the pie for 5 minutes, then leave to cool completely before slicing.
Serve cold, as a snack or store in the fridge for up to 2 days.
Recipes and photographs taken from Danube by Irina Georgescu, photography by Issy Croker, (Hardie Grant, £28)
