Heat oven to 200C/180C fan/gas 6. Put the celeriac and potatoes in a pan, cover with salted water and bring to the boil. Reduce heat and simmer until just tender, about 5 mins. Drain, then toss with cream, rosemary and seasoning. Set aside.
Roll one pastry block 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 (chopped into chunks), then repeat the layers until all the ingredients are piled up.
Roll out the second pastry block large enough to cover the bottom sheet and filling with some overhang. Brush the bottom border with a little egg, then drape the pastry sheet over, pushing down to seal the edges. Trim off excess with a sharp knife and crimp the edges together with a fork. Can be made a day ahead and chilled before baking or frozen for up to 1 month. Defrost 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. Bake for 25 mins until golden brown.
©author BBC Good Food Magazine. Produced as part of the LOVE BAKING season on BBC Lifestyle channel 180 on DStv.