(Serves 6-8)

Cooking time 1 hour 15 minutes, plus 1 hour chilling

Ingredients

For the pastry:

425g self-raising flour, plus extra for dusting

120g unsalted butter, chilled and cubed, plus extra for greasing

1/2tsp salt

25g walnuts, ground

25g Pecorino cheese, grated

25g Parmesan cheese, grated

1 egg yolk, lightly beaten

100-150ml iced water

Beaten egg, to glaze

For the filling:

1 potato, peeled and cubed

1 small sweet potato

3 large onions, sliced

50ml milk

2tbsp plain flour

200g Lancashire cheese, or Cheddar cheese, grated

1tbsp mascarpone cheese

1tsp English mustard

1tsp Worcestershire sauce

1tsp Tabasco sauce

Salt and pepper

1. Put the flour, butter and salt into a food processor. Pulse until it resembles fine breadcrumbs. Add the ground walnuts and cheeses, pulse briefly, then the egg yolk and pulse again. Add the water a few drops at a time until the dough comes together.

2. Cut off one-third of the dough for your pie lid. Flatten both pieces into discs, wrap in clingfilm and chill for an hour.

3. Meanwhile, make the filling. Cook the potato cubes in a saucepan of salted boiling water for about 10-15 minutes until tender. Drain and set aside.

4. Pierce the sweet potato with a fork. Cook in the microwave on high for 8-10 minutes, turning once. Cut in half, scoop out the insides and mash.

5. Put your onions and milk into a microwave-proof bowl. Cook on High for 8-10 minutes, stirring halfway through.

6. Tip the onions and milk into a large saucepan over a medium heat. Sprinkle over the flour, stir and cook off the flour for a few minutes. Stir in the potatoes, and then the rest of the filling ingredients. Set aside.

7. Preheat the oven to 200C (400F), Gas Mark 6. Grease and flour a 20cm round pie dish.

8. Roll out the larger pastry disc on a floured surface until large enough to line your dish. Use the back of your finger to press your pastry base into the edges of the dish. Trim off any excess and prick your base. Line with non-stick baking paper, fill with baking beans (or uncooked rice) and bake for 15 minutes.

9. Remove from the oven, lift out the paper and beans, then spoon in the filling. Roll out the smaller piece of pastry to fit the top of your pie. Brush beaten egg around the rim of the pastry base, gently lay your lid on and press the edges with a fork to seal. Trim off any excess and make a slit in the middle of your pie.

10. Give it a good wash of beaten egg and bake for 30 minutes. Let it sit for at least 30 minutes.