Fresh and smoked salmon with choucroute en croûte

An Alsace classic for September, from:

FRESH AND SMOKED SALMON WITH CHOUCROUTE 
EN CROUTE, BEURRE BLANC
 This makes a great dish for entertaining as the whole thing can be prepared in advance ready for baking. If you can get hold of the fish heads and bones for stock, so much the better (simmer them for 30 minutes with water to cover, a bouquet garni and an onion). You don't really need an accompaniment, but fresh pasta or tiny rattes potatoes go nicely.
  Serves 6-8 generously300g  cooked choucroute
3-4 tbsp crème fraîche
2 x 600g skinless, boneless fillets of salmon or sea trout 
100g thinly sliced smoked salmon
salt and pepper
500g puff pastry
1 egg, lightly beaten
Beurre blanc
3 shallots, finely chopped
150ml dry white wine
1 tbsp vinegar
250 ml  well reduced fish stock
250ml whipping cream
salt and pepper
100-150g butter, cut in pieces

• Squeeze out the choucroute if it seems rather wet, mix in the cream
• Season the fish lightly, run your finger down the flesh going towards the tail and remove any extraneous bones with tweezers 
• Cut the pastry in two pieces, one weighing 300g, the other 200g
• Roll out the larger piece to an oval shape somewhat bigger than the fish 
• Put one of the salmon fillets on top, followed by half the choucroute
• Lay smoked salmon slices on top, finish with choucroute and the other salmon fillet
• Press the edges of the pastry up against the sides of the fish, brush the exposed edges with water
• Roll out the smaller piece of pastry and cut it to a fish shape to fit on top
• Lay it on top of the fish, pinching the pastry edges together to seal
• Snip the pastry with scissors to simulate scales and use trimmings to decorate
• Line a baking sheet with baking parchment, glaze the fish with egg and refrigerate if not to be baked immediately
• Heat the oven to 220oC and bake the fish for 30-35 minutes or until the pastry is golden and the fish just done
• Meanwhile make the sauce: put the shallots in a small pan with the white wine and vinegar and boil hard to reduce to barely a tablespoon
• Add the fish stock and reduce by half
• Whisk in the cream and reduce again
• Check the seasoning, adjusting if necessary
• Remove from the heat and whisk in the butter in small pieces - the sauce will thicken and emulsify
• Slice the fish (easier said than done - use a very sharp knife or electric carving knife) and pour the sauce around it