Fit the K-Beater.
Add plain flour, wholemeal flour, butter and salt into the Appliance bowl, fit the splashguard.
Mix for 2 minutes on speed 2, until the mixture resembles breadcrumbs.
Add water and beaten egg.
Mix for 1 minute on speed 1, until the pastry comes together.
Wrap the pastry in plastic wrap and chill in the fridge for 2 hours.

Clean the Appliance bowl and fit the Stir Tool.
Add water and salt into the Appliance bowl, fit the splashguard.
Heat for 2 minutes at 160ºC, on speed Stir 2, until the salt has dissolved.
Add potatoes.
Cook for 10 minutes at 105ºC, speed Stir 3, until softened.
Drain potatoes into a colander, then transfer into a clean bowl and set aside.

Clean the Appliance bowl and fit the Stir Tool.
Add butter into the Appliance bowl.
Heat for 1 minute at 120ºC, on speed Stir 1, until the butter has melted.
Add onions, leeks and salt, fit the splashguard.
Cook for 15 minutes at 120ºC, on speed Stir 2, until translucent.

Remove the Stir Tool and fit the K-Beater.
Add potatoes back into the Appliance bowl.
Add whipping cream, thyme, salt and pepper.
Mix for 1 minute on speed Min, until combined.
Transfer the mixture into a clean bowl.
Let cool for 30 minutes.
Then stir half the cheddar cheese into the mixture.

Place the chilled pastry onto a lightly floured work surface.
Roll out the pastry using a rolling pin into a circle about 5 mm thick.
Line a round pie dish (25 cm) with the pastry leaving some overhanging the sides.
Spoon the filling into the pie dish and spread evenly.
Top with the remaining cheese.
Trim the edges, then brush pastry with egg yolk to glaze.
Chill uncovered in the fridge for 30 minutes.

Preheat the oven to 220ºC.
Preheat a baking tray in the oven.
Remove the pie from the fridge and place on the preheated baking tray.
Bake for 40 minutes at 220ºC, until golden brown.
Remove from the oven and let rest for 20 minutes.

Serve.