Roll out half the pastry on a lightly floured work surface to form a 25 x 35 cm (10 x 14 inch) rectangle. Repeat with the remaining pastry. Cover with clean tea towels and leave to rest.
Melt the butter in a saucepan and gently fry the shallots and lemon rind for 3 minutes. Stir in the flour and cook for 30 seconds. Remove from the heat, stir in the cream and then heat gently, stirring continuously, for 2 minutes until thickened. Remove from the heat and season with the nutmeg, salt and pepper. Cover the surface with clingfilm and set aside to cool.
Drain the spinach well and season with a little salt and pepper. Lay 1 piece of pastry on a large baking sheet lined with baking paper and spread the spinach over the top, leaving a 2.5 cm (1 inch) border at each end and a 5 cm (2 inch) border down each side.
Stir the salmon, prawns and tarragon into the cooled cream mixture and spoon over the spinach. Brush the edges of the pastry with water and top with the other piece of pastry, pressing the edges together firmly.
Trim the pastry to neaten and then press the edges firmly together to seal. Brush with the beaten egg and pierce the top to allow the steam out.
Bake on a pre-heated baking sheet in a preheated oven, 220°C (425°F), Gas Mark 7, for 20 minutes, then reduce the temperature to 190°C (375°F), Gas Mark 5, and bake for a further 15 minutes until the pastry is risen and golden.