Grate about 500g beetroot (peeled and trimmed) into a large mixing bowl.   Add a finely chopped onion, season.  To make the dressing whisk together 150ml olive oil, 150ml cider vinegar (or wine vinegar), 1 tsp mustard, salt (use celery salt if you have some) and ground black pepper. Tip the dressing over the beetroot and onion and stir to coat the vegetables.  Cover and refrigerate before serving – it can be left for up to 24 hours.  The longer it is left the more the flavours mingle together.