Easy and cheap (was a staple during my student days!) - Layered potato bake:
Slice one or two large potatoes (about the thickness of a pound coin)
In a casserole/oven-proof dish, layer the potato slices with whatever veg you have to hand.
Onions are pretty essential, finely chopped garlic is also good, but anything else can go in there.
Season each layer with salt/black pepper and finish with a layer of potato.
Then take a can of Campbells condensed soup - mushroom or celery work very well - mix it with a can full of water or milk, and then pour all over the potato.
Cover with foil (or lid) and bake in the oven till the potato is soft.
Also nice if you take off the foil/lid and add some grated cheese on the top, and cook till this is browned/bubbling.