Peel and halve onions and beetroot. Place them on a baking tray with the cut surface facing down. Cook in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 1 hour. Peel, wash and chop the potatoes. Cook in boiling salted water for about 20 minutes.
After 30 minutes cooking time turn onions and beetroot. Drain potatoes and let them evaporate. Add 20 g butter and milk to the potatoes and mash to puree. Season with salt and pepper. 15 minutes before the end of the cooking time, place mashed potatoes on the vegetables in portions and sprinkle with cheese and herbs. Place 30 g butter in flakes on top