1. Preheat oven to 190?C/gas mark 5.
2. Bring a saucepan of water to the boil and cook potatoes until tender – about 12-15 minutes. Add the leeks in the last 3-4 minutes of cooking time. Drain well and mash with the natural yoghurt. Beat until smooth and fluffy.
3. Place the fish pieces and milk in a large wide saucepan and poach the fish gently for 5 minutes. Scoop out using a slotted spoon and transfer to an ovenproof dish. Tuck in the spinach leaves.
4. Return the pan of milk to the heat and add the corn flour paste. Heat gently, stirring continuously until the sauce begins to thicken. Add the mustard, dried herbs or parsley and season with black pepper. Stir in half the grated cheese and simmer for 2 minutes.
5. Pour the sauce over the fish and spinach leaves. The leaves will wilt a little from the sauce.
6. Top with mashed potato and sprinkle over the remaining grated cheese.
7. Bake in the oven for 25 minutes until the top is golden brown.
900g (2lb) white potatoes, peeled, chopped into even sized pieces
2 tablespoons low fat natural yoghurt
1 medium leek, sliced and washed
700g (1½lb) mixed white fish and salmon, skinless, chopped into large pieces
568ml (1 pint) semi-skimmed milk
200g spinach leaves, washed
1 heaped tablespoon (30g) corn flour, mixed to a paste with 1 tablespoon water
1 teaspoon wholegrain mustard
1 teaspoon dried herbs or 1 tablespoon fresh chopped parsley
Black pepper, to season
150g (5oz) mature cheddar cheese, grated
2 tomatoes, chopped