Dry the prawns, if frozen and thawed, by patting between sheets of kitchen paper. Season the cornflour and use to coat the prawns and white fish. Lightly crush the peppercorns using a pestle and mortar.
Put the peppercorns in a food processor with the fennel, leek, dill, parsley and a little salt and blend until very finely chopped, scraping the mixture down from the sides of the bowl if necessary. Tip into a shallow, ovenproof dish.
Scatter the prawns and fish over the fennel mixture and mix together a little. Scatter the peas on top.
Spoon half the cheese sauce over the filling and spread roughly with the back of a spoon. Layer up the potatoes on top, seasoning each layer as you go. Spoon the remaining sauce over the top, spreading it in a thin layer. Sprinkle with the cheese.
Bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 30 minutes until the surface has turned pale golden. Reduce the oven temperature to 180°C (350°F), Gas Mark 4, and cook for a further 30–40 minutes until the potatoes are completely tender. Serve with a tomato salad.