Preheat the oven to 180°C / 350°F / Gas mark 4.
Line a baking tray with greaseproof paper and set aside.
Heat a large pan of water, add the potatoes and cook for 8 minutes until just soft.
Add the peas 1 minute before the end of cooking time. Drain, then set aside.
Meanwhile, in a pan, heat the oil and fry the onion until lightly browned.
Add the curry paste and mustard seeds, then fry for a few minutes more until the mixture smells fragrant.
Carefully stir in the potatoes and peas, trying not to mash them.
Add the lemon juice and coriander, then leave to cool.
Unroll the pastry rectangle, place on a floured surface and roll it out a little more to make it into a square shape.
Cut into 4 squares, then cut each one in half so you have 8 long rectangles.
Place 4 of them onto the baking sheet, brush the edges with the beaten egg and place a quarter of the filling down the centre of each.
Top with the remaining pastry, then pinch the edges together to make a sealed parcel.
Brush with more egg. These can be frozen at this point.
Bake for 20 minutes until puffed and golden.
If cooking from frozen, bake for 30 minutes.
Serve with fresh green salad and a spoonful of chutney if desired.