Place the rice into a pan of boiling water with a pinch of turmeric and cook for 15-17 minutes, or until just done but still firm, then drain.
Melt the butter in a frying pan and fry the onion for 4-5 minutes, until soft.
Add the mushrooms and fry for 3-4 minutes, or until softened.
Add the cooked rice, salmon, parsley, tarragon, eggs and season, to taste, with salt and freshly ground black pepper.
Place the puff pastry onto a clean flat surface. Cut out a rectangle 30cm/12in x 20cm/8in.
Spoon the salmon mixture down the length of the middle of the pastry rectangle, then wrap the pastry around the mixture.
Brush the pastry all over with the beaten egg, then place onto a baking tray and place in the fridge to cool for 30 minutes.
Preheat the oven to 200C/400F/Gas 6.
Sprinkle the pastry parcel with sesame seeds and transfer to the oven to cook for 30 minutes, or until hot throughout and crisp and golden-brown all over.