Zest 3 of the lemons and juice all of the lemons. You should have about ¾ cup of lemon juice.
In a bowl, whisk together the lemon juice, grated zest, sugar, salt and the eggs until well beaten.
In a saucepan, melt the butter.
Add the lemon juice combination.
Cook on medium low heat and simmer for 10 minutes, stirring frequently, until thickened.
Transfer to a glass jar and leave to cool.