1. Simmer the rice in a large saucepan of boiling salted water for 12-14 minutes, or until cooked (follow the instructions on the packet). Drain and set aside to cool.
2. Melt half the butter in a large frying pan over a medium-to-low heat and cook the shallots for about 10 minutes, or until soft, but not coloured. Add the chopped mushrooms and cook for about 5 minutes, then stir in the lemon juice and zest. Transfer to a large bowl.
3. Heat the remaining butter in the frying pan and add the salmon. Cook over a gentle heat for 3-4 minutes, or until just cooked. Flake the salmon into bite-size pieces and transfer to a second bowl. Add the herbs, chopped egg and half the crème fraîche and gently combine. Season to taste.
4. Combine the rice with the remaining crème fraîche in a third bowl. Unroll one of the blocks of pastry and lay it on a large baking sheet that has been lined with baking paper. Trim the pastry into a large fish shape. Leaving a 2cm (¾in) border, spread the pastry with a layer of the rice mixture (you may not need all the rice). Follow with the salmon mixture, then the mushrooms, seasoning as you go. Unroll the other pack of pastry and lay over the filling. Press the pastry edges together firmly and trim. Chill for at least 30 minutes, but you can refrigerate it for up to 12 hours.
5. Combine the rice with the remaining crème fraîche in a third bowl. Unroll one of the blocks of pastry and lay it on a large baking sheet that has been lined with baking paper. Trim the pastry into a large fish shape. Leaving a 2cm (¾in) border, spread the pastry with a layer of the rice mixture (you may not need all the rice). Follow with the salmon mixture, then the mushrooms, seasoning as you go. Unroll the other pack of pastry and lay over the filling. Press the pastry edges together firmly and trim. Chill for at least 30 minutes, but you can refrigerate it for up to 12 hours.
6. Heat the oven to 200ºC (gas mark 6). Using a small sharp knife, cut scales and decorations into the pastry, being careful not to cut all the way through. Brush with the beaten egg and bake for 30-40 minutes, or until golden and risen. Cut into slices and serve with lemon butter sauce.