Heat oven to 200C/180C fan/gas 6.
Put the celeriac and potatoes in a large
pan, cover with salted water and bring to
the boil. Turn down the heat and simmer
until just tender, about 5 mins. Drain,
then toss with the cream, rosemary
and seasoning. Set aside.
Roll one of the blocks of pastry until
just bigger than a large dinner plate, cut
into a neat circle and place onto a flat
baking sheet. Leaving a 3cm border,
spread out a layer of celeriac and
potatoes, top with some cheese and
bacon, then repeat the layers until
all the ingredients are piled up.
Roll out the second block of pastry
large enough to completely cover the
bottom sheet and filling with some
overhang. Brush the bottom border with
a little of the egg, then drape the pastry
sheet over, pushing down to seal the
edges. Trim off the excess with a sharp
knife and crimp edge together with a
fork. Can be made a day ahead and
chilled before baking or frozen for up to 1
month. Defrost completely before baking.
Pierce a steam hole in the top of the
pie, brush all over with remaining egg and
score the pastry lightly with spoke marks,
if you like. Bake for 25 mins until golden
brown. Stand for 5 mins before serving.