Clean and gut the fish. Heat the oil in a flameproof casserole and add the tomatoes, onions, thyme, salt and pepper. Simmer over a low heat for 15 minutes. Mix the roes and breadcrumbs in a bowl. Add 1 tablespoon of the tomato and onion mixture, mix well and season. Stuff the fish with this mixture and place on top of the remaining mixture. Add the cider.