Preheat the oven to 200C/400F/Gas 6.
Prepare the base of the pie. Cut off one-third of the pastry. Wrap the larger piece in cling film and place in the fridge (this will be used to make the top of the pie later). On a clean, floured surface, roll out the smaller piece of pastry into a rectangle about 20cm x 28cm/8in x 11in. Carefully trim the edges to form a neat rectangle. Reserve the trimmings and place in layers that can be rolled out to make the decoration later.
Using a rolling pin to lift it, place the pastry rectangle onto a non-stick baking tray and prick all over with a fork. This will stop it from rising too much in the oven. Bake in the oven for 12-15 minutes until cooked and golden. Set aside to cool.
For the filling, heat half of the butter in a frying pan until foaming. Finely chop the shallots and fry until soft and golden-brown. Add the sliced mushrooms to the shallots and fry until just cooked. Remove from the heat and leave to cool.
Place the salmon fillets into a bowl. Squeeze over the lemon juice. Tear the dill into pieces and scatter over the salmon. Drizzle over the olive oil, then season with freshly ground black pepper and gently combine. Don't prepare the filling more than an hour in advance, or the lemon will be begin to 'cook' the salmon.
In a bowl, combine the cooled shallot and mushroom mixture with the cooled rice. Season to taste with salt and freshly ground black pepper. Spoon the mixture over the cooked pastry base and, using the back of a spoon, press the mixture out toward the edges.
Place the salmon slices and dill on top of the rice (discard the juices from the bowl). Splash over some white wine and dot over the remaining butter.
On a clean, floured surface, roll out the remaining pastry until it's slightly larger than the base. It should be large enough to cover the mixture and the cooked pastry with a little room to spare. Using a rolling pin, gently lift and place the pastry over the salmon and trim off the corners of the raw pastry. Reserve for later.
Using a palette knife gently tuck the raw pastry underneath the base (a bit like making a bed).
Make an egg wash by beating together the egg and salt in a bowl. Using a pastry brush, brush the whole pie with the beaten egg. Use the reserved pastry trimmings to make leaf shapes and decorate the pie with the leaves. Brush over these with beaten egg.
Bake the pie for about 30-35 minutes, or until golden-brown. Remove from the oven and allow to cool for 5-10 minutes before serving.
Meanwhile, make the soured cream. Place the cream into a bowl. Pour over the lemon juice and stir thoroughly as the cream thickens. Season with freshly ground black pepper.
Serve slices of the salmon en croûte with the soured cream on the side.
