Place egg and soda water into the Thermomix bowl and mix 10 seconds/speed 2.
Add plain flour and cornflour and mix 5 seconds/speed 3 until just combined. Scrape sides of bowl and stir through with spatula. Do not over-mix; a few small lumps are fine. Tip mix into a shallow bowl.
In a second shallow bowl, add enough flour to coat the onion rings.
Slice onions 1-centimetre thick and separate rings.
Heat olive oil in a deep pan until temperature reaches 180°C.
Coat each ring in plain flour, then dip 2–3 rings at a time into the batter until thoroughly coated.
Once the rings have stopped dripping, carefully place into the hot oil. The rings will almost immediately float to the surface so they will need to be turned once or twice during cooking to brown on both sides.
Cook until golden brown then place on a plate lined with paper towel and immediately sprinkle with salt flakes. Keep warm in a low oven until ready to serve.
You do need to keep an eye on the temperature of the oil when frying. It will fluctuate as you add food and take it out etc. So just maintain the 170°C-180°C range as best you can.