For the dough, knead 150 g flour, salt, butter in pieces and 3-4 tbsp. cold water to a smooth dough. Form dough into a ball, wrap in cling film and chill for about 30 minutes
Peel the onion and cut into fine strips. Heat the oil and braise the onion for about 5 minutes until soft. Peel, wash and thinly slice the potatoes. Peel celeriac and grate roughly. Cut bacon into strips, add to the onions, dust with 1 tbsp. flour and stir in
Put half each of the potato slices, celery and onion-bacon mixture one after the other into a greased round ovenproof dish (20 cm Ø). Season with salt and pepper. Repeat the process with the remaining ingredients. Mix cider and mustard and spread over the filling
Roll out the dough into a circle (22 cm Ø) on a floured work surface. Whisk egg yolk and 2 tbsp. water. Coat the edge of the mould with a thin layer of the mixture. Cover the mould with the rolled out dough and press down the edge. Spread the dough with the rest of the egg yolk
Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for approx. 1 1/4 hours. After half of the time, cover with aluminium foil if necessary