Heat oven to 200C, 180C fan, gas 6 with a baking tray inside. Thinly roll out the pastry on a lightly floured surface until large enough to line a 22 - 23cm round, loose-bottomed tart tin. Lift onto the tin with the aid of your rolling pin, then tuck snuggly into the base and sides leaving the excess pastry overhanging the top of the tin. Prick the pastry base with a fork. Line the tin with a sheet of baking parchment and baking beans then bake blind on the hot baking tray for 15 minutes. You can use ceramic beans, or dried beans which can be cooled and stored for another time. Remove the paper then bake for 5 -10 minutes more to dry the base of the pastry. Carefully trim away the overhanging pastry with a sharp knife. To make the filling, put the butter, flour and milk in a pan and heat gently, stirring all the time with a wooden spoon or ballon whisk until it makes a very thick, smooth sauce. Cool for a few minutes then beat in the egg yolks, dill, lemon zest, and seasoning. Whisk the egg whites in a clean bowl with an electric hand whisk until they hold their shape then fold carefully into the sauce - the mixture should look light and fluffy. Tear the smoked salmon slices into large pieces and arrange half of it the tart tin with half the goat?s cheese. Spoon the sauce on top then dot over with the remaining salmon and cheese. Bake for 25-30 minutes until risen and golden.