Preheat the oven to 200°C.

Skin the potatoes and cook them for 25 minutes in boiling salted water. Then cut the potatoes into pieces. Skin the apples and cut them into pieces. Skin the onions, cut them into rings and steam them with a little bit of butter. Slice the cheese.

Line a pie plate with puff pastry. Spread the potatoes, the onions, the apples and the cheese. Cover the pie with the second puff pastry. Press the edges firmly. Spread the lid with egg and pierce the lid several times with a fork.

Bake the pie in the preheated oven at 200°C for about 40 minutes.