Lightly sauté the onions in the butter until tender, allow to cool. Beat the egg lightly and remove a little for the glaze, then whisk the crème fraiche, lemon juice and rind into the rest of the egg, season well.
Unroll one of the pastry sheets and lay on a lined baking sheet. Leaving a border of approx. 2cm all round, layer up the ingredients in an even stack, spooning the egg/ crème fraiche mixture on every now and then, and sprinkling with the chopped herbs as you go.
Unroll the second pastry sheet and make 5 crescent shaped cuts radiating out from the centre but keeping the centre point intact. Brush the base pastry border with beaten egg, top with the second pastry sheet and press the edges firmly together to seal. Crimp or roughly flute the edges if you wish, brush the top with beaten egg and sprinkle with poppy seeds if desired. Bake for approx. 20 minutes until pastry is a deep golden brown.
To give it a little moisture we have added some lightly sautéed onions, a little egg & crème fraiche or cream mixture with lemon juice.