Grind the salt with the seeds in a pestle and mortar until the salt is finely ground. Transfer to a sterilised 2 litre jar with a lid and add the honey, ginger and flour ( or rye starter).
Pour in the warm water and whisk until the honey, salt and flour have dissolved. Cut the beetroot into quarters or 5 mm dice and add to the jar. put a lid on the jar and leave at room temperature The kvas will froth up the next day and should be ready in 3 days. It should be a little fizzy and smell and taste pleasantly sour.
If you are going to use it within a couple of days you can pour it straight from the jar but if you want to keep it for longer strain through a sieve into a jug and pour into three sterilised 700 ml bottles. Store in the fridge or a cold place, where it will keep for up to three months.
You can reuse the beetroot and ginger to make another batch of kvas. Just add the same amounts of salt, spice, honey, flour or starter and water as above.
Drink the kvas in a similar way to kombucha or use it in broths and stews to add a sour note.