Boil the potatoes in a large pan of salted water for 15-20 minutes until soft.
While the potatoes are cooking, melt the butter in a small frying pan and gently fry the onions for 5-10 minutes until soft and transparent. Try not to let them get too much brown colour.
Preheat the oven to 200℃ fan/220℃/Gas Mark 7/400F fan.
When the potatoes are cooked, drain them and add the onions (and any butter left in the bottom of the pan), along with the cheese and plenty of salt and pepper. Mash well and allow to cool.
Roll out the puff pastry and cut into 8 rectangles.
Share the potato mixture across 4 of them. Brush around the edges with beaten egg and sit the other 4 ‘lids’ on top.
Seal the edges using a fork and brush the top with the remaining egg.
Place on a baking sheet lined with baking paper/parchment (or use the sheet that the puff pastry came on).
Bake for 15-20 minutes until very golden.
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Calories: 797kcal | Carbohydrates: 66g | Protein: 17g | Fat: 53g | Saturated Fat: 19g | Polyunsaturated Fat: 5g | Monounsaturated Fat: 24g | Trans Fat: 0.2g | Cholesterol: 47mg | Sodium: 494mg | Potassium: 584mg | Fiber: 5g | Sugar: 4g | Vitamin A: 505IU | Vitamin C: 24mg | Calcium: 257mg | Iron: 3mg