Place unopened can of evaporated milk in a saucepan and cover with cold water. Simmer for 30 minutes. Take care to ensure that the water does not totally evaporate; top up as needed with hot water from the kettle.
Cool can under cold water and leave in fridge unopened overnight. It can stay in fridge for a couple of days.
Add milk and sugar to bowl, and whisk with electric whisk for 5 minutes, until all sugar lumps have gone. Whilst whisking slowly add all the lemon juice. The mixture should thicken and all lemon juice should be incorporated.
Divide mixture between the two baked pastry cases and leave to set for 1 hour at room temperature (do not put in the fridge).