For the lemon tart filling

• Into a large bowl pour the condensed milk, the zest from 3 of the lemons, a pinch of salt and the juice from all 4 lemons onto the egg yolks.

  Bring to the boil on low heat and simmer until thickens.
• Take it off the heat, filter everything through a strainer and let cool. Put in the fridge for 1 hour.

  Heat the oven after 30 minutes to 180 °C (350 °F).
• After cooling, mix lemon filling well with a balloon whisk and pour into the prepared pastry case.

  Bake for 30 minutes.
• Let cool and serve dusted with icing sugar or caramelise the top with brown sugar.