01 -
Combine plain flour, baking powder, salt, butter, and parmesan in a food processor and pulse until the mixture resembles breadcrumbs. Add the egg yolk and cold water, pulsing to form a dough. Shape into a disc, wrap in cling film, and refrigerate for 1 hour.
02 -
Place diced potato in a saucepan, cover with cold water, and bring to a boil. Simmer until just tender, then drain and set aside.
03 -
Boil sliced onions in water for 2–3 minutes. Drain thoroughly, return to the pan, and stir in plain flour.
04 -
Add milk and double cream to onions. Cook over low heat, stirring continuously until thickened.
05 -
Add grated cheddar, mustard, cayenne, and cooked potato to the onion mixture. Season with salt and pepper. Stir well and allow filling to cool completely.
06 -
Preheat oven to 180°C. Roll out two-thirds of the chilled pastry to line a pie dish. Trim excess and lightly prick the base with a fork.
07 -
Line pastry base with parchment and fill with baking beans. Bake for 10–15 minutes until edges are lightly golden. Remove beans and paper; leave to cool.
08 -
Spread the cooled cheese and onion filling evenly into the pastry case. Roll out the remaining pastry and cover the filling. Trim and seal the edges, then crimp to decorate.
09 -
Brush the top with beaten egg to glaze. Bake at 180°C for 25–30 minutes, or until the pastry is golden and crisp.
10 -
Allow to rest for 10 minutes before slicing and serving.