To make the pastry, sieve the flour into a large mixing bowl and add 1 tsp sea salt. Coarsely grate in the cold butter, then use your fingertips to work into the flour until it resembles breadcrumbs. Pour over the vodka and use a butter knife to mix it in, then pour over 2 tbsp cold water and stir again with the knife. If you need to, add a little more water until it comes together as a dough, about 1-2 tbsp. Lightly knead the pastry to a smooth dough in the bowl, dust lightly with flour and wrap in baking parchment. Rest for at least 30 mins.
Peel and cut the potatoes into 5mm- thick slices and put them in a lidded saucepan. Cover with cold water, add a generous pinch of salt, bring to the boil with the lid on, then lower to a simmer and cook for 8-10 mins until just tender. Drain and leave to steam-dry.
In a large non-stick frying pan, heat the olive oil, bay leaf and butter together over a medium heat, then add the leek and onion. Season well and add a good grating of nutmeg. Cook, stirring, for 8-10 mins, until softened. Add about 100ml water, and cook until most of it has evaporated. Discard the bay leaf and stir in both the mustards and parmesan. Set aside to cool. Butter and flour a 20cm round pie dish.
Roll ¾ of the pastry out on a lightly floured surface in a circle larger than the pie dish, then line the dish. Roll the rest into a large circle for the lid and chill both for 10 mins. Heat the oven to 200C/180C fan/gas 6.
Spoon a layer of the leek mixture into the lined dish, grate over some cheddar and top with a layer of potatoes and the melting cheese slices. Repeat the layering until you have filled the pie dish – press down to compact it.
Brush some of the egg around the edges, top with the pastry lid and crimp together. Egg wash the lid, scatter over some sea salt and cut two slits in the lid. Bake for 45-50 mins until golden.
Remove from the oven and allow to sit for 10 mins, then slice and serve with a watercress salad, if you like.