Boil the beetroot until soft when poked with a knife. It took about 40 minutes for me one day and less the next, so give it plenty of time or even do it the day before. Meanwhile, wrap each fillet of salmon in parchment and bake in the oven at 180°C for 20 minutes until cooked through. Then drain the beetroot and mash them well together with the butter and horseradish sauce - this can be done with a masher or even in a food processor. Season well and serve with the salmon.