Wash potatoes thoroughly and cook in boiling salted water for about 20 minutes. Drain the potatoes and place them carefully side by side on a baking tray lined with baking paper, sprinkle with sea salt and bake in a preheated oven (electric cooker: 225 °C/ convection oven: 200 °C/ gas: see manufacturer) for approx. 30 minutes.
In the meantime, clean the radishes, pluck the leaves from the stems and put them aside. Wash the radishes and cut into fine strips. Peel and halve the onion and cut into fine strips. Peel garlic and chop very finely. Stir curd, lemon juice, sugar and milk until smooth. Stir in radishes, except for a few for sprinkling. Season to taste with salt and pepper. Wash the radish leaves and shake dry.
Heat the oil in a pan. Fry the onion and garlic for about 1 minute while turning. Add the radish leaves and steam for about 1 minute. Season to taste with salt and pepper. Arrange the potatoes on plates, cut them deep lengthwise and fill them with some curd cheese. Sprinkle with radishes and add steamed radish leaves. Add the remaining radish quark.