Prepare all the fish, shell the prawns after cooking in boiling water for 3-4 minutes, skin the monkfish and remove the central bone, skin the cod and clean, cut into chunks at least 1 inch cubes. Sprinkle with lemon juice and zest.
In a large fry pan, melt the fat and olive oil but do not allow to burn. Add the leeks, spring onion and cook gently for 2 minutes. Next add the cubes of fish, prawns and toss gently in the pan being careful not to break up the texture. Cook for 4 minutes approximately turning gently. Pour over the liquid if using cream the sauce should thicken on its own, if using milk you may need to blend it with al little (1oz) cornflour or flour to thicken the sauce.
To prepare the champ, drain and mash the peeled cooked potatoes, add the butter, milk, seasoning and beat well to form a smooth champ.
Transfer the fish pie mixture into a serving dish, add 2 tsp pesto sauce, sprinkle with peas, chopped herbs such as dill, fennel or parsley, top with potatoes. Smooth on top and bake in the oven @ 375°F/gas no 5 for 20-25 minutes approximately.
Serve hot garnished with lemon and herbs.