Step 1: Prepare the pastry

• In a mixing bowl, rub the butter into the flour until the mixture resembles breadcrumbs.
• Add cold water, one tablespoon at a time, until the dough comes together.
• Wrap the dough in cling film and chill in the fridge for 30 minutes.

Step 2: Cook the potatoes

• Peel and chop the potatoes into chunks.
• Boil them in salted water for 15–20 minutes until tender. Drain and mash with butter, milk, salt, and pepper. Set aside.

Step 3: Caramelize the onions

• Heat butter in a frying pan over medium heat.
• Add the sliced onions and cook for 10–12 minutes until golden and caramelized.

Step 4: Combine the filling

• Mix the mashed potatoes with the caramelized onions and grated cheese. Taste and adjust seasoning as needed.

Step 5: Assemble the pie

• Preheat your oven to 200°C (400°F).
• Roll out half of the pastry on a floured surface and line the pie dish.
• Spoon the potato, onion, and cheese mixture into the pastry shell.
• Roll out the remaining pastry and place it on top, sealing the edges. Trim any excess and crimp the edges with a fork.

Step 6: Bake the pie

• Brush the pastry with beaten egg for a golden finish.
• Bake in the preheated oven for 25–30 minutes, or until the crust is golden brown and crisp.

Step 7: Serve

• Allow the pie to cool slightly before slicing and serving. Pair with a crisp green salad or steamed vegetables for a complete meal.