Cabbage gratin

I have seen many recipes for vegetable gratins, but not one that includes cabbage. Why that should be, I don't know: cabbage works just as well in this context as chard, say, or spinach, particularly with a strong cheese such as Cheddar.

This recipe involves my new favourite method of cooking cabbage. For two.

1 onion, chopped
Butter and a little sunflower oil, for frying the onion
Half a cabbage, such as Savoy, chopped and washed
About 35g butter
2dstsp flour
About 350ml milk
100g Cheddar, grated
Grating of nutmeg
1tsp Dijon mustard
2tbsp Parmesan, grated

Heat the oven to gas mark 8/230C. Warm a gratin dish. (I put mine into the grill section above the oven.)

Cook the onion, over a gentle heat, in enough butter to prevent its catching (the addition of a little oil is helpful). Give it five to 10 minutes to soften and start to turn golden. Throw in the wet cabbage, and cook, on a slightly higher heat and stirring regularly, until the leaves have wilted and softened - another five to 10 minutes. (The green colour deepens.)

Meanwhile, melt a large knob of butter in a small saucepan, and add the flour. (Use more butter if this roux is too thick.) Cook the roux, very gently, for about a minute - this process helps to soften the floury flavour. Turn up the heat to medium, and add the milk, a few splashes at a time, incorporating each before adding the next. Stop when you have a fairly thick sauce (which liquid from the vegetables will thin). Bubble it for a minute or so, and add the Cheddar, nutmeg, and mustard, with a little salt. (Remember that the cheese is salty.)

Stir the sauce into the cabbage and onions, and tip the mixture into the warm gratin dish; sprinkle the surface with Parmesan, and put the dish into the hot oven for five minutes. (You're just giving it a blast of heat. There's no need to cook it for any longer.) Brown the surface under the grill, if you like.