To make the dough, in a large bowl combine the flour with the cayenne pepper, parmesan, vegetable powder and salt. Add the cubed butter and rub the mixture with your hands until it looks like breadcrumbs. This could take a while. Pour in half the egg, 2 Tbsp water and the yogurt and knead until you have a smooth dough. Cut ⅓ of the dough and flatten it with a rolling pin. Do the same with the remaining dough. Put the two dough pieces on a tray, cover with cling film and keep in the fridge for at least 30 mins.

Preheat the oven to 200°C.

While waiting for the dough, prepare the filling. Heat the butter in a large saucepan. Add the onions, leeks, bay leaves, thyme, vegetable powder, mustard powder, grainy mustard, salt and pepper. Cook on low heat for 15 mins until the onions have softened and caramelized. Add the potatoes and continue to cook, covered for 10 minutes. Add the double cream, mix and turn the fire off. Remove the bay leave and add the cheeses.

Roll out the big piece of the dough to line a round pie dish covering the sides as well. Add the mixture inside and even it out. With the remaining dough, cut out festive shapes to decorate your pie. Brush with the remaining egg and bake in the oven for 35 minutes until golden.

Serve at room temperature!