Preheat the oven to 200ºC/400ºF/gas 6. Scrub the celeriac, rub with 1 tablespoon of oil and wrap in tin foil. Roast for 1 hour 30 minutes, then finely slice and season with sea salt and black pepper. Meanwhile, halve, wash and finely slice the leeks, then place in a large casserole pan on a medium heat with the butter. Peel, finely slice and add the garlic and mushrooms, then cook for 15 minutes. Stir in the flour and mustard, followed slowly by the milk, then simmer for 5 minutes, or until thickened, stirring regularly, and remove from the heat. Pick, finely chop and stir in the parsley, crumble in the cheese, then season to perfection.
Line a 20cm bowl (8cm deep) with clingfilm. Arrange slices of celeriac in and around the bowl until covered. Reserving half the sauce, layer up the rest with the remaining celeriac in the bowl, finishing with celeriac. Pull over the clingfilm, weigh it down with something heavy, and chill overnight with the remaining sauce.
Preheat the oven to 180ºC/350ºF/gas 4. On greaseproof paper, roll both sheets of pastry out to around 30cm x 35cm. Unwrap the filling parcel and place in the middle of one sheet. Beat the egg and brush around the edge of the pastry and all over the celeriac, then carefully lay the second piece of pastry on top, smoothing around the shape of the filling. Trim the edges to 2.5cm, crimp to seal, then eggwash all over. Very lightly score the pastry (like in the picture), making a small hole in the top. Bake at the bottom of the oven for 2 hours, or until beautifully golden, brushing with more eggwash once or twice, then serve with the warmed-up creamy sauce. Delicious with dressed seasonal steamed greens.