Wash the potatoes, dab dry and wrap in aluminium foil. Cook in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 1 hour. In the meantime, clean and wash the radishes and cut them into sticks.
Wash, halve, core and dice the cucumber. Wash chives, dab dry and cut into fine rolls. Mix quark, olive oil, radish, cucumber, chives and parsley. Season with salt, pepper, sugar and lemon juice.
Remove potatoes from the oven and serve with the quark dip. Garnish with lemon and chives.