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
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.