Preheat the oven to fan 160C/conventional 180C/gas 4. Trim the beetroot, scrub well and pat dry with kitchen paper. Tip into a roasting tin and toss in 1 tbsp of the oil. Bake for 2 hours until tender. You can check by giving them a squeeze – if cooked they will feel slightly soft. Set aside.
Sprinkle the surface of the meat with the sea salt and freshly ground black pepper. Heat the remaining oil until very hot in a large frying pan, add the meat and sear on all sides until evenly browned. Transfer to another roasting tin and roast for 40-50 minutes for medium.
Remove from the oven, set on a board, cover tightly with foil and leave to rest for 10 minutes. While the meat is resting, return the beetroot to the oven to reheat for 10 minutes. Meanwhile, mix the horseradish with the crème fraîche and season with salt and pepper.
To serve, thinly slice the beef and arrange on warmed plates. Cut each beetroot almost in half and set beside the beef. Spoon horseradish cream on top and sprinkle with parsley and chives. Serve with crushed potatoes with rocket and lemon.