This is a variation on Nigel Slater's blue cheese and potato pie. It works particularly well at this time of the year, because the new potato skins add interest to the texture. Any hard cheese would suit -- as would any blue one. A generous portion for two.
600g new potatoes, scraped
4 onions, each cut in half then cut into chunks
150g Gruyere, grated
Put the potatoes into cold water in a saucepan, bring slowly to the boil, and simmer gently until tender. Drain.
Meanwhile, fry the onions in enough oil to prevent their catching on the pan, until soft and golden. About 20 minutes.
Roughly mash the potatoes with a fork, leaving them slightly lumpy. Stir in the onions and cheese. Add a little salt (the cheese is salty), and pepper according to taste. You could grate over a little nutmeg if you like.
Tip into a warm gratin dish, and bake in a gas mark 6/200C oven for about 15 minutes.