Boil the water in a kettle and allow it to cool slightly.
Place the mustard seeds into the Thermomix bowl and mill 10 sec/speed 10. Tip out and set aside.
Without rinsing the bowl, pour the boiling water into the Thermomix bowl. Add the beetroot and allow it to steep 10 min/speed 1 without any heat.
Add the remaining ingredients and stir 3 sec/speed 5. Pour into a large glass jar or jug and cover with a paper towel or muslin cloth. Leave in full sunlight or a warm place for 4-5 days until well fermented. Shake it or stir it regularly.
Strain and chill it before drinking. This drink will last up to a week in the fridge, but the flavours lessen over time.
You can do a second ferment with this drink for some natural fizziness. Pour the strained liquid into pop top bottles and leave on the kitchen bench for up to 3 days. Burp the lids every day. Once you know they are nice and fizzy, refrigerate before consuming.