Ingredients

Step by step

1. For the pastry, peel and roughly chop the potatoes and place in a saucepan of cold salted water, bring to the boil and cook for 15-20 minutes until tender. Drain, run under cold water to cool then drain again. Place on a plate lined with kitchen paper and chill. Once the potatoes are cold, put the flours and salt into a large mixing bowl and rub in the fats until the mixture resembles breadcrumbs. Mash the potatoes and add 225g to the pastry (any leftover can be added to the pie filling), along with the egg white. Bring the dough together until it forms large clumps; it will look quite dry at this point; just leave it for 5 minutes to let the flours absorb the moisture, then bring into a ball of dough. Knead briefly; shape one third of the pastry into a disc, wrap and chill.
2. Grease and flour a 20cm springform or loose-based deep cake tin (at least 6cm deep). On a floured surface, roll out the remaining pastry to a 35cm round and use to line the base and sides of the tin. Leaving a little overhang, trim off any excess pastry. Chill in the fridge.
3. In a large, deep frying pan, heat 1 tablespoon of oil and the butter over a medium heat. Add the onions with a pinch of salt and gently cook for about an hour until soft and caramelised, stirring every so often. Leave to cool.
4. Meanwhile, peel and dice the potatoes for the filling into 2cm cubes. Add to a saucepan of cold salted water, bring to the boil and cook for 8-9 minutes until just tender. Drain, refresh under cold water and drain again. Spread out on a clean tea towel and leave to dry out.
5. In the same saucepan, gently fry the garlic in 1⁄2 tablespoon of olive oil for 1-2 minutes, then add the spinach, cover and leave to wilt. You may need to add it in 2 batches. Season to taste with salt, pepper and nutmeg. Leave to cool then squeeze out as much moisture as possible. Season the potatoes well and stir together with the caramelised onions and the chopped herbs (plus any leftover mash from the pastry).
6. Place a baking tray in the oven and preheat to 200°C, fan 180°C, gas 6. Remove the pastry disc and lined tin from the fridge. Scatter a third of the dried breadcrumbs (or oats) over the base of the pastry-lined tin, then layer as follows; a third of the potato, a quarter of the cheese, half the spinach, a quarter of the cheese and a third of the breadcrumbs. Repeat these layers, then finish with the remaining potatoes.
7. Roll the pastry disc into a circle slightly larger than the diameter of the tin. Brush a little cold water around the overhanging edges of pastry and lift the disc on top of the pie, pressing down to seal. Trim away any excess, then crimp all the way around. Brush the top of the pie with the beaten egg yolk and make a criss-cross in the pastry lid to allow steam to escape. Use the leftover pastry to decorate, if you wish, and glaze again.
8. Slide the pie onto the hot baking tray in the oven and bake for 1 hour, covering with foil for the final 20 minutes, once golden. Leave to cool completely in the tin then chill in the fridge until ready to serve. Remove from the tin, slice and serve with piccalilli or a chutney.