Heat the milk into a fairly large saucepan, without letting it boil. Then add the lemon zest.
Open the vanilla bean and scrape out the vanilla. Add the vanilla and the vanilla bean to the milk.
Separate the egg yolks from the whites and whisk together in a bowl with the sugar.
Once light and fluffy, stir in the sifted flour. Once well mixed, slowly add the egg and flour mixture to the hot milk, mixing as you go.
Once you have added all the milk, place the saucepan over low heat and cook, stirring with a whisk or wooden spoon, until the cream is thick enough.
Remove the saucepan from the heat and pass through a chinois, if there are lumps.
If you don’t plan to use the pastry cream immediately, place it in a bowl and cover it with parchment paper so that a solid membrane forms on the top due to air contact.