Place the flour, sugar, lemon juice, lemon zest, and water in a saucepan.
Whisk constantly. About 2 to 3 minutes after the mixture begins to boil, it will start to thicken. Lower the heat and temper the egg yolks: take a ladleful of the hot mixture and slowly whisk it into the yolks. Repeat 2–3 times, then pour the tempered yolks back into the saucepan.
Tip: Be sure to stir the lemon filling mixture occasionally to prevent it from sticking to the bottom of the pan.
Tip: Tempering the eggs is very important, otherwise you risk ending up with scrambled yolks!
Next, add the butter and stir until the lemon filling is smooth. Transfer the filling to a bowl, cover with plastic wrap, and let cool in the refrigerator.