Boil the fish with salt and 1 tbsp lemon juice and keep aside
Fry onions in butter and oil in a saucepan until soft
Add tomatoes, tomato sauce, salt, pepper and lemon juice
Chop or flake the cooked fish and add to the mixture and mix well
Add 1/4cup water, bring to the boil and turn mixture into an oven-proof dish
Prepare mashed potatoes as per instructions with the addition of salt, white pepper, lemon juice and butter
Mix well and spread over the fish mixture
Beat an egg and brush over the potatoes, sprinkle with breadcrumbs and cook in an oven at 200°C (Gas Mark 6 or 400°F) for 15 minutes