Wash the fish slices neatly under running water. Place them on a big plate and marinate them with the lemon juice for 10 minutes. The lemon juice keeps the flesh firm together during cooking. After marinating dry the fish steaks and salt them. Slightly flour the fish on both sides. Spread the oil over the bottom of a baking dish which is big enough for all the slices. Arrange the fish slices and spread oregano, pepper and salt over them. Top it off with the butter cut into small flakes. Pour little water into the form, around one centimeter deep. Put the fish steaks in the oven and cook at the lowest possible heat for 30 minutes. Towards the end of the cooking time spread sauce from the bottom of the dish over the fish steaks to avoid drying out. If your oven does not provide really low heat, reduce the cooking time for about 10 minutes. Serve with boiled potatoes, vegetables or steamed rice.