Potato, Bacon, And Celery Root Pie - cooking recipe

Ingredients

5 tbsp butter, diced
1 cup, plus 2 tbsp all-purpose flour
1 tbsp sunflower oil
1 None large onion, peeled and cut into thin strips
7 slices bacon, cut into strips
1 3/4 lbs potatoes, peeled and sliced
1 1/3 lbs celeriac (celery root), peeled and grated
1 cup dry hard cider or dry white wine
1 tbsp wholegrain mustard
1 None egg yolk, beaten with 2 tbsp water

Preparation

In a bowl, knead the butter, 1 cup plus 1 tbsp flour, a pinch of salt and 3-4 tbsp cold water to a smooth dough. Shape into a ball, wrap in cling film and chill for 30 mins.
Preheat the oven to 400\u00b0F. Heat the oil in a skillet and saute the onion for 5 mins, until softened. Add the bacon, sprinkle with 1 tbsp flour and stir. Spoon into an 8-inch round pie dish, add the potatoes and celeriac and mix well. Season with salt and black pepper. Mix the cider and mustard together and pour over the pie filling.
On a lightly floured work surface, roll out the dough to an 8 1/2 inch diameter circle. Lay over the filling, pressing around the edges. Brush with the egg yolk. Bake for 1 hour 15 mins, covering with kitchen foil halfway through cooking if the crust is browning too quickly.