Bring a pan of salted water to the boil, then cook the potatoes for 8 minutes or until just tender but not falling apart. Drain well and leave in the colander to steam dry.
Soften the sliced onions in the butter in a frying pan until really soft but not golden. Remove from the pan to cool.
Unroll the pastry sheet onto a big baking sheet. Trim a strip from the shorter end and press to join on one of the longer sides so it becomes more like a square, less like a rectangle. Cut a 25cm circle from the pastry (you can roll it a little to increase in size, if you need to).
Place a base layer of potato slices on top of the pastry, leaving a 3cm clear border around the edge. Next layer up the rest of the ingredients distributing the soft onions, ham, peas, parsley, cheddar and seasoning among the potatoes to make a big dome of filling on top of the pastry.
Roll out the pastry block on a lightly floured surface to a circle around 40cm wide. Brush the border of the pie base with a little beaten egg, then use your rolling pin to carefully lift the pastry to cover the pie base. Press the pastry down the sides, squeezing out any air, to stick down on the border. Trim the excess pastry, and poke a steam hole in the very top. Crimp or fork the border to seal, then brush the whole pie with beaten egg. Chill for 30 minutes, or up to a day.
Preheat the oven to 190°C (170°C fan oven) Gas Mark 5. Bake the pie for 50 minutes, then sprinkle with a little more cheddar and bake for 10-20 minutes more until the pastry and cheese are golden. Serve warm or cold with salad, mustards and pickles.