Heat the oven to Gas 8, 230°C, fan 219°C. Grease a baking tray.
Heat the butter in a small pan over medium heat and fry the shallot for 3 minutes. Add the flour and cook, stirring for 1 minute. Add the cream and nutmeg and cook for 1 minute, stirring, until thickened. Add the feta cheese and tarragon and allow the cheese to melt a little. Remove from the heat and add the spinach. Stir well.
Unroll the pastry and cut with a knife into 2 equal pieces. Place one piece of pastry on the greased baking tray and spread the spinach mixture on top keeping it 1cm (½in) within the edge. Brush the edge of the pastry with beaten egg and lay the second piece of pastry on top. Carefully press the edges together to seal and then mark with a fork all around. Brush the top of the pastry with beaten egg and make a few holes in the top with the fork.
Bake in the oven for 20 minutes or until puffed and golden.
Tip: For a change add three rashers of finely chopped bacon when you fry the shallots.
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