In a large preserving pan or heavy pot, combine the diced beetroot, onions, water, vinegar, salt, and spices.
Bring to the boil, the turn it down to simmer for 30-40 minutes
Once the beets are almost cooked through, add the sugar and stir until it has dissolved. Cook for an additional 10-20 minutes, until the beetroot is tender and fully cooked.
To thicken, combine the cornflour and 4 Tbsp water and slowly add it to the beetroot. Stir constantly and it will thicken up and turn all glossy and thick.
Follow the water bath canning instructions below if you want to can it. Or just follow instruction six below.
Spoon hot relish into sterilized jars ( I use my dishwasher) seal immediately, and allow to cool.
Store in a cool, dark place — the flavour deepens beautifully after a week or two.