For the crème pâtissière. Put the milk and vanilla pod in a large pan, bring to boil, then turn off the heat. In a large bowl, whisk the yolks and sugar for 10 minutes until voluminous, then beat in the flour and lemon zest. Strain the warm milk slowly into the egg mixture, whisking continuously. Clean the pan, then pour the mixture back in, then gently bring to the boil, stir until thickened. Take the pan off the heat, cover the surface of the crème pâtissière with baking paper and leave to cool.