Directions

1. Bring a pan of salted water to the boil, then cook the potatoes for 8 mins until just tender but not falling apart. Drain and leave to steam-dry in the colander.
2. Meanwhile, heat the butter in a wide pan. Stir in the onions, seasoning and the thyme, then cover and cook for 8 mins until translucent and turning golden. Peel and slice the apples, toss with the lemon juice, then cook with the onions in the now uncovered pan for 5 mins, stirring often. Mix in the potatoes, then leave to cool. Remove the thyme.
3. Cut just under two-thirds of the pastry from the block, squash into a round, then roll out on a flour-dusted surface until large enough to line a deep 23cm loose-bottomed tart tin, leaving a little excess overhanging. Fill the pastry case with layers of the potato and apple mix, and all but 1 tbsp of the cheese. Dot with the rest of the butter, if using, and season generously. Brush the overhanging edge of pastry with egg. Heat oven to 190C/170C fan/gas 5 and put a baking tray in the oven to heat up.
4. Shape the remaining pastry into a round, then roll out until just large enough to cover the top of the pie. Press the edges together. Trim the pie by pressing your thumb against the rim of the tin all the way around. Brush with egg, then cut slits to allow steam to escape. Slide the pie onto the hot baking sheet and cook for 1 hr, scattering with the remaining. cheese with 10 mins to go, until the pastry and cheese are golden. Serve warm or eat cold with the pickled cabbage or beets on the side.