Begin by drying the liver slices with kitchen paper, then lightly coat the pieces in the seasoned flour. Next heat the butter and oil together in a frying pan and soften the sliced onion in it over a low heat for about 10 minutes.
Now arrange half the floured liver slices in the base of a deep ovenproof casserole and cover with a layer of onion, carrots and scraped new potatoes, followed by half the sliced tomatoes. Then repeat all over again, finishing up with a layer of tomatoes. Combine the stock, oregano and tomato puree together in a jug, season, pour this into the casserole, cover closely and bake for about 11/2 hours, or until the vegetables are tender.