Lisbon lemons are a great choice of lemon when it comes to making the lemon tart filling. If these can’t be sourced then I recommend you use another lemon with similar tartness such as Eureka lemons. Sweeter lemons such as Meyer lemons can also be used but then you’ll need to reduce the amount of sugar called for in this recipe otherwise you will upset the sweet and tart balance of the filling (unless you prefer a much sweeter lemon tart!).