1. Preheat oven to 200C/400F/Gas 6.
2. Melt half the butter in a large saucepan and cook the leeks for 2-3 minutes until soft.
3. Stir in the flour.
4. Remove from the heat and gradually stir in the milk. Return to the heat and cook stirring until the sauce has thickened. Gently simmer for 5 minutes.
5. Stir in the prawns and season with salt and pepper.
6. Place half the sauce in a pie dish; add the fish and spoon over the remaining sauce.
7. Top with the sliced potatoes and dot with the remaining butter.
8. Cut the tomato and place on the top of the pie along with the grated cheese so both grill and melt with the potato topping.
9. Place the dish on a baking sheet and bake for 35-40 minutes until the potatoes are golden.
10. Serve with peas and salad garnish.