Cut the prawns in half lengthways and set aside. Cut the bread in cubes and soak in the milk. Preheat the oven to 120°C (250°F/Gas ⅓). Cut the salmon into thin strips and set aside. Blanch the broad beans in boiling water for 3 minutes and rinse under cold water. Slice the mushrooms and fry briefly on both sides in a frying pan. Sprinkle with salt and pepper and set aside. Separate the egg whites from the yolks. Cut the white fish into chunks and add them to the food processor bowl. Add the drained, soaked bread and egg yolks and blend into a coarse purée. Pour the purée into a bowl and add the cream. Add the prawn halves and the chives. Beat the egg whites until stiff in another bowl and carefully stir through the white fish mixture to keep it airy. Season the mixture with salt and pepper. Generously line an oblong cake tin with plastic wrap and allow the edges to hang over. Cover the bottom with half the salmon strips. Pour half of the white fish mousse over the salmon. Arrange the mushroom slices and broad beans on the fish. Cover with the remaining mousse. Top the mousse with the rest of the salmon. Cover the terrine with the overhanging plastic wrap and place in a larger oven dish. Fill that bowl with water halfway up the terrine and bake in the oven for 75 minutes. The terrine must feel firm when pressed, otherwise briefly return to the oven.
First leave the terrine to slightly cool then place in the refrigerator for at least 12 hours. Serve with horseradish sauce. Or green herb oil. Or with a small green salad and mustard dressing.