This recipe is based on one in Nigel Slater's Kitchen Diaries. He advised 225 g Stilton for six people; we shared 175 g (I should guess) between two. There is quite a lot of saturated fat here.
4 medium potatoes
3 red onions, sliced
30 g butter
175 g blue cheese (I used St Agur), cut into small chunks
I slice white bread, whizzed into breadcrumbs
Salt and pepper
Peel and cut up the potatoes, cover them with cold water, bring to a simmer, and cook until soft. Drain, and return them to the hot pan to dry for a few minutes. Mash, or pass through a vegetable mill, with a little salt.
Meanwhile, melt the butter in a heavy saucepan over a low heat, tip in the onions, and cover. Stir them from time to time. The onions should throw off enough water to stop them sticking. Cook for about 25 minutes, until soft.
Spoon half of the mashed potato into a buttered gratin dish. Cover with the onions, then the cheese, grind over black pepper, and top with the rest of the mash. Cover with breadcrumbs, and bake at gas mark 6/200 C for 30 minutes.