Wash the beetroots well and wrap in foil. Bake in a preheated oven, 150°C/300°F/gas mark 2, for 1 hour. Alternatively, boil the beetroots in lightly salted water for 1 hour, until tender. When they are cooked the skin can be rubbed easily from the root. Grate the cooked beetroot. Reheat thoroughly in the butter, sharpening with vinegar to taste. Season well with salt and pepper, and serve.