In a pan heat the butter and add the onion and fry until soft and golden.
Mix the cream with the egg, mustard and season with black pepper.
In a 23-25cm pie dish layer up 4 sheets of buttered filo in the base, alternating directions and allow the excess to overhang the sides.
Now add half of the sliced potato, scatter over some onion, then spinach and then half the cream and cheese.
Season with salt and pepper and repeat with the remaining potato, spinach and onion and season again with salt and pepper and top with remaining cream and cheese.
Fold over the overhanging filo and then top with a further 2-3 sheets of buttered filo.
Brush the top of the pie with butter.
Bake for 10 minutes at 210c/190c fan then cover the top with a sheet of foil to prevent burning and turn the oven down to 190/170c fan and cook for a further 20-25 minutes until heated right through.
Allow to cool in the tin for about 5-10 mins before serving with green beans or green salad.
