I make lots of vegetable stuff, that must be really healthy, because it's nearly all vegetable, with very little fat or anything.
One of my current favourites is butternut squash.
Butternut squash gratin:
Chop butternut squash (skin included) into pieces about a cm thick. Layer in a dish or casserole dish that can go in the oven. Then put a layer of red onion (sliced in rings) on top. Then another layer of squash, then another layer of onion. Sometimes I stick little bits of butter in the layers too, but it's not necessary. Plenty of black pepper and a bit of coarse sea salt over the top, then stick in the oven (about 150-160 degrees C) until the squash is getting soft.
Then chop brown/seedy/granary bread (better if a bit stale) finely, and chop lots of parsley, mix together and layer on top of the squash and onions, and put back in the oven for another ten minutes or so.
If you want to be really healthy, add some sunflower seeds and pumpkin seeds for a bit of crunch. If you are allowed a bit of fat (and I always add this, because we need a BIT of fat, plus cheese is good for you, calcium etc) then I grate red leicester or mature cheddar all over the top. The bread and cheese makes a nice crusty topping.