Peel the potatoes, cut into even-sized pieces and boil until tender. Drain and mash with 50g (2oz) butter and season to taste.
Heat the remaining butter in a frying pan and sauté the fennel over a low heat for 5 minutes or until it softens but doesn´t turn brown. Stir in the peas, prawns, salmon and cheese sauce and spoon into an ovenproof dish.
Preheat the oven to 200ºC / 400ºF / Gas mark 6. Spoon the mashed potatoes over the fish mixture, pressing down gently in an even layer. Lift the pie on to a baking sheet and bake for 30 minutes or until the potatoes are golden.