01 -
Combine flour and kosher salt in the bowl of a food processor. Pulse briefly to mix. Add diced cold butter and pulse repeatedly until mixture resembles coarse crumbs with small pea-sized pieces.
02 -
Gradually pulse in ice water until the dough forms small clumps that hold together when pressed. Transfer dough to a work surface, press into a mass, divide in half, and shape each into a flat disc. Wrap with plastic wrap and chill for at least 1 hour.
03 -
Peel sweet potatoes. Thinly slice both sweet and yellow potatoes to 3 mm thickness using a mandolin or food processor. In a measuring jug, whisk together cream, flour, chopped sage, salt, nutmeg, and pepper until smooth. Set aside.
04 -
Preheat oven to 175°C. Lightly grease a 23 cm pie plate. On a floured surface, roll one chilled dough disc into a circle about 5 cm larger than the pie plate. Fold in half, transfer to the pie plate, and unfold, allowing excess to drape over the edge.
05 -
Arrange a third of the potato slices, alternating sweet and yellow, in the crust. Layer with a third of the cheddar, and drizzle with a third of the cream mixture. Repeat layers twice more with remaining ingredients.
06 -
Roll out remaining dough disc into a circle slightly wider than the pie plate. Brush bottom crust edge with beaten egg. Place top crust over filling, press edges to seal, trim excess, and crimp for a decorative finish. Brush top and edges with more egg. Cut several slits in the crust for steam venting.
07 -
Set the pie on a baking tray. Bake at 175°C for 90–105 minutes, until crust is golden and potatoes are tender. Protect crust from over-browning with foil if needed. Test doneness by sliding a paring knife into steam vents.
08 -
Allow pie to cool for 15–30 minutes for easier slicing and serving. Portion and enjoy warm.