The fish are cut to length and placed in the water. Peel the onion, cut into larger pieces and pour into a deep dish, which is mixed with tomato paste and water. In the pan, greased with sunflower oil, pour half of the onion and tomato puree. Top rank drained fish inside up. On fish pour the other half of the onion. Add salt and bake in a moderate oven. Serve with lemon slices.