This quiche Lorraine is ambiguous: thought to be as French as Edith Piaf, its origin and name (related to the german Kuchen) is German.  Regarded an easy dish, this version is anything but. It is based on the recipe found in the book ‘Heston Blumenthal at Home’ which makes it by definition challenging to execute. But do not be discouraged, it will be an instant hit at your dinner parties. Plus it can be prepared a day in advance as this helps the custard filling to set properly. For the best flavour, it should be served warm or at room temperature. Serves 6 or hungry 4. For the pastry 230g all purpose flour 1/2 tsp sea salt 100g cold unsalted butter, cubed 25g egg, lightly beaten (approx. 1/2 large egg) 40g cold tap water For the filling 40g unsalted butter 4 large onions, peeled  and finely sliced 200g bacon lardons 3 large eggs 300g whipping cream 80g grated gruyère cheese 20g grated parmesan cheese sea salt and freshly ground black pepper nutmeg For the pastry put …