For two people -
Four big flat mushrooms (we use Portabella I think)
About half a red onion
(optional) - can of chopped tomatoes, courgette
If you're using the can of tomatoes and courgette, just thinly slice the courgette lengthways with a peeler and put that and the tomatoes into the bottom of a pyrex dish, stirring so the courgette and tomatoes are mixed.
Place the mushrooms on top, brown frilly side up, and thinly slice the red onion over it. Slice the cheese and arrange on top to cover, and add some black pepper. Pop it in the oven for about 20 minutes on 200 degrees.
Alternatively don't use the chopped tomatoes and courgette - use a sliced tomato or two and arrange it on top of the mushrooms with the onion, under the cheese. Bake it for the same amount of time.
Goes very well with chicken/fish/roast meat/everything :)