Preheat oven to 170 degrees celsius (fan-forced). Grease an ovenproof casserole/baking dish and set aside (minimum of 2 litre/8cup capacity).
Place the caster sugar, butter and grated lemon zest into the Thermomix bowl and mix for 10 seconds on Speed 5. Scrape down the sides of the bowl and repeat for a further 20 seconds or until pale and creamy.
With the blades turning on Speed 3, add in the egg yolks one by one through the MC hole.
Add the lemon juice, plain flour and full cream milk through the MC hole and continue mixing for 10 seconds or until combined. Set aside.
In a completely clean and dry Thermomix bowl, insert the butterfly and beat the egg whites on Speed 3.5 until firm peaks have formed. Gently mix through the lemon mixture with a spoon.
Pour into the prepared casserole/baking dish. Place the dish into a large pan and fill halfway up the side with boiling water (to create a water bath around the baking dish).
Bake for 30 minutes or until lightly golden and firm on the top and a sauce has formed underneath (it should be wobbly at the bottom).
Dust with icing sugar and serve while warm with cream or ice-cream.