Place the flour, icing sugar, butter and egg yolks into the Thermomix bowl. Mix on Speed 8, 10 seconds then on Interval/Knead function for 2 minutes.
Knead dough into a ball, wrap is plastic wrap and place into the fridge for 10 minutes.
Press the dough evenly into a round 30cm flan tin with a removable base. Trim any overhanging pastry from around the top. Place the pastry tin onto a flat tray (this makes it easier to move) and place into the freezer for 20 minutes.
Preheat oven to 180 degrees celsius (fan-forced).Prick the base of the pie with a fork and then blind bake the base for 15 minutes (see notes) or until golden. Remove from the oven.
To make the lemon filling, place the caster sugar into the Thermomix bowl and mill for 10 seconds, Speed 9. Scrape down the sides of the bowl.
Add the cornflour, lemon zest, lemon juice, water and butter and cook for 4 minutes, 60 degrees, Speed 2. With blades still rotating on Speed 2, add the egg yolks and whole egg one at a time through hole in mixing bowl lid. Once eggs are mixed through, cook for 7 minutes, 80 degrees, Speed 2. The mixture will thicken on cooling.
Spread the lemon filling over the top of the cooked pie base.
To make the meringue, insert the butterfly into a clean and dry Thermomix bowl and add the egg whites. Set the timer for 8 minutes, 37 degrees, Speed 3.5. When the meringues form soft peaks, very slowly add the cornflour and then the caster sugar through the MC hole. Continue to beat until firm peaks form.
Place large spoonfuls of the meringue around the edge of the lemon meringue pie and then work your way into the middle with the remaining meringue. Use a knife to add a few swirls.
Bake in the oven for 20 minutes or until golden on top.
Allow to cool in the tin for 30 minutes before removing and leaving to cool for a further 1 hour (minimum) before serving.