quiches, soups... one of my faves is leftover pie which is any left over roast meat and veg from the day before's meal, all packed in layers in a shortcrust or lardcrust pastry thn baked. it's great cold for picnics. it's extra speacially good if the left over meat is lamb. but that's just personal opinion!!
I love bubble and squeak.
And Fadge. yeah i know, stupid name. it's left over boiled or mashed potato (you're gonna mash it anyway) and rolled oats. put the mash in a bowl, add 1 egg yolk (or 2tsp arrowroot dissolved in 1 dstsp boiling water if you are not into eggs), half a finely chopped onion or a couple of scallions and stir well. sprinkle on oats and keep adding oats until the mix forms a dough. roll out and cut into rounds or form into balls in your hands and flatten. fry in a little oil until brown all over. they are even more scrummy if you use other mashed root veg in them.