Preheat your oven to 200°C/Fan 180°C/Gas 7. Begin by making the pastry. Tip the flour into a bowl, setting 2 tbsp aside for later. Coarsely grate in the butter. Rub the flour and butter together to create a breadcrumb-like texture. Finely grate in half the cheddar and lightly rub that in.
Beat the egg. Pour half of it into the butter and flour. Mix together with a fork. Trickle in 1-2 tbsp cold water till you have a smooth, but not too wet dough. Lightly knead. Wrap the dough in cling film and pop it into the fridge to rest.
Trim the roots and tops off the leeks, then slice them into 1cm-thick rounds. Slice the mushrooms. Heat a large frying pan for a couple of mins. Drizzle in 1 tbsp oil. Add the leeks and mushrooms. Fry for 5 mins till the veg have softened slightly.
While the leeks and mushrooms are cooking, finely shred the parsley leaves. Zest the lemon. Finely grate the remaining cheese.
Add the parsley to the pan. Sprinkle in 1 tbsp of the reserved flour and stir to evenly coat the veg. Slowly pour in the milk, stirring constantly. Add 1 tbsp mustard and the grated cheese. Season generously. Stir together well. Simmer for 5 mins.
While the pie filling is simmering, dust your work surface with the remaining 1 tbsp plain flour. Unwrap the pastry and roll it into a large thin piece the shape of your pie dish, about ½ cm thick.
Taste and adjust the seasoning of the mushroom pie filling if necessary. Spoon it into a pie or baking dish. Cut the rolled pastry to cover the pie. Tuck the edges into the sides of the filling. If you have any excess pastry left, you can cut shapes to decorate your pie lid. Brush the top of the pastry with the remaining beaten egg, using it to stick on any decoration. Sprinkle over some black pepper.
Slide the pie into the oven for 20 mins till the top is golden and crisp. Meanwhile, finely shred the kale. Heat ½ tbsp oil in a large frying pan or wok. Add the kale and stir fry for 2-3 mins till tender. Add the lemon zest. Squeeze over a little lemon juice and season.
Remove the pie from the oven and cut it into large slices. Serve with the stir-fried lemony kale.