Cut two thirds of the salmon into thin pieces and set aside.
Purée the remainder in a food processor.
Add the yolks and 70g butter and process again and with the motor running add the egg white and then gradually add the cream.
Stir in the grated nutmeg, salt and pepper and put the bowl of the processor in the fridge while you prepare the spinach.
Remove the tough stalks and plunge into boiling water until wilted, approximately 30 seconds. Remove from the heat and drain.
Roll out the pastry into 2 circles one larger than the other. Take a 20cm cake tin approximately 10cm high, flour this well and place the larger pastry circle into it with a little pastry overlapping the sides of the tin.
Put a pancake on the pastry base, spread one quarter of the salmon mixture on the pancake then a half of the salmon pieces and another layer of salmon mixture.
Arrange a thin layer of spinach on top, then cover with another layer of the mixture, the salmon pieces and a final layer of mousse. Place a second pancake on top. Brush the edges of the pasty with beaten egg and place the second circle of pastry on top.
Place onto a baking tray, remove the tin and brush with the egg. Bake for approximately 50 minutes.