Peel and grate the beetroot. Place in a pan and cover with water. Peel the ginger and chop finely or, even better, pound it with a pestle and mortar and add to the mixture in the pan. Cover the pan and leave for a few hours. After a while, strain the liquid from the grated beetroot. I've found the best way to do this is to lift out a handful of grated beetroot and squeeze the liquid from it back into the pan. Throw away the squeezed beetrot and repeat the process. Once you're down to just a few bits of beetroot floating around, strain the liquid through a seive, into a large jar. Add the cider vinegar and your tonic is ready to drink. It will keep in the fridge for a few days.