1. Preheat the oven to 220°C.
  2. Place the squares of puff pastry next to each other on a Griddle and baking tray lined with grease-proof paper.
  3. Beat the egg yolk with 2 tbsp of water and brush the pastry with it.
  4. Bake the puff pastry in the centre of the oven for about 12 minutes until golden and done.
  5. In a wide pan, bring the fish stock and the wine to the boil and leave the mixture to reduce until you are left with about 3 tbsp. 
  6. Sprinkle the lemon juice over the fish fillet, season with salt and pepper, and place the fillet in a steamer insert.
  7. Bring water to the boil in a saucepan, making sure there is at least 1 inch between the water and the steamer insert.
  8. Place the steamer insert with the cod fillet on the saucepan above the boiling water and cover. Steam the cod for 8-10 minutes until done. 
  9. In the meantime, mix the sour cream with the reduced liquid and let the sauce simmer for about 5 minutes. 
  10. Stir the tarragon into the sauce and adjust the seasoning.
  11. Mix the olives and dried tomato in a bowl.
  12. Divide the cooked fish in pieces of about 1 inch and spoon them carefully into the olive and tomato mixture. 
  13. With a serrated knife, cut the slices of puff pastry open as you would a roll. Place the bottom halves on the plates and divide the fish mixture over them.
  14. Spoon over the sauce and place the puff pastry lids on top. Delicious with steamed mange-tout.