Bring the milk to the boil with the vanilla pod and lemon rind in it; then leave it to cool down. In the meanwhile, beat the egg yolks with the sugar until light and fluffy. Stir the flour into beaten yolks, a little at a time, making sure it is well amalgamated. Then very slowly pour in the cold milk (from which you have removed the vanilla pod and lemon rind). Put the mixture back on the stove, over a very low heat and, stirring constantly with a wooden spoon, bring the pastry cream almost to the boil, removing it from the heat just before it begins to do so. If any lumps should have formed while cooking, whisk the pastry cream slightly until it’s smooth and homogenous.