Heat the oven to 375F/190C/Gas Mark 5-6. Slice the cooked potatoes thinly, grate the cheese. Grease a 2 1/2 pt/1l pie or shallow ovenproof dish well and put in a layer of potatoes, cover with half the cheese and then put another layer using up the remaining potato. Beat the eggs, stir in the milk, season with salt, pepper, and nutmeg, and snip in the parsley. Pour the mixture over the potatoes. Top with the remaining cheese and dot with the butter. If serving as a supper dish on its own, decorate the top with sliced tomatoes.
Cook in the hot oven for 20-25 minutes, until the custard is set and the top is golden and bubbly. Serve as it is for an accompaniment, or top the cooked dish with the sliced hard-boiled egg and washed watercress sprigs if eating on its own.