I'm sure most vegetarians would be really happy to be served a slice of this. And it would sit nicely at the table for a buffet, while a slice would be great with roast lamb.
A couple of good knobs of butter
1 medium onion, peeled, halved and finely chopped
1 x 400-500g celeriac, peeled
Salt and freshly ground black pepper
250-300g ready-made, all-butter puff pastry
Plain flour, for dusting
200g Lancashire cheese, grated
1 egg, beaten
Melt the butter in a pan, add the onion and gently cook for 2-3 minutes without colouring. Remove from the heat and leave to cool. Cut the celeriac in half and slice it as thinly as possible with a very sharp knife or mandolin. Blanch the slices in boiling, salted water for 2-3 minutes.
Roll out the pastry on a lightly-floured surface to a 1cm thickness and cut it out into two discs, one about 25cm across and the other about 30cm. Lay the smaller one on af tray and prick holes in it with a fork. Arrange a layer of the celeriac slices over the pastry, leaving a 2cm margin around the edge.
Scatter over some onion, a little cheese and season, then add another layer of celeriac slices. Continue layering up the ingredients until you have used them all up.
Brush the edges of the pastry with egg and lay the larger pastry disc on top, pressing the edges together with your fingers. You can decorate the pithivier by making lines with the back of a small knife from the centre of the pastry if you wish. Brush with egg and leave to rest in the fridge for 30 minutes. Preheat the oven to 180C/gas mark 5.
Bake the pithivier for about 30-40 minutes until golden. Serve.