Blend the beetroot and onion together in a food processor or blender. Set aside.
Heat a little oil in a covered frying pan and fry the mustard seeds until popping. (CAUTION: when the mustard seeds pop, keep the pan well away from your face and eyes.)
Add all the ingredients and 125ml/4fl oz water to a large deep frying pan or large saucepan. Place over a medium heat, cover and bring to the boil.
Cook for 30-60 minutes, or until the beetroot is soft and the liquid has reduced and thickened to the desired consistency.
Remove any excess liquid with a ladle as necessary, and remove the clove. Pour into sterilised jars, seal and let cool.