Preheat oven to 160 degrees celsius (fan-forced). Grease and line an 18X28cm slice tin with baking paper and set aside.
Melt the butter in the Thermomix on 80 degrees, Speed 2, 3 minutes (or until melted).
Add the biscuits and crush on Speed 8, 5 seconds. Scrape down the sides of the bowl and repeat for a further 5 seconds.
Firmly press the mixture into the base of the prepared tin and bake for 10 minutes. Remove from oven and allow to cool.
Place the sweetened condensed milk, lemon juice and eggs into a clean and dry Thermomix bowl and mix on Speed 4, 45 seconds. Scrape down the sides and mix for a further 45 seconds or until creamy. Note: the mixture will be very runny at this stage.
Pour the liquid into the prepared base and bake for 15 minutes or until set.
Allow to cool in the fridge for a minimum of 4 hours (preferably overnight).
Store in an airtight container in the fridge for up to 5 days.