Wash the maties thoroughly, dab dry and place in a mould. Peel onions and cut them into rings. Wash lemon hot, dab dry, also cut into slices. Bring vinegar, 1/4 litre water and the prepared ingredients with the peppercorns to the boil.
Season with sugar, let it cool down a bit and pour it over the maties. Wash the lemon balm, cut it into strips and sprinkle over it. Cover and leave to stand overnight. Shortly before serving, wash the potatoes thoroughly, dab dry and cut in half lengthwise.
Heat the oil in a frying pan and fry the potatoes for about 15 minutes, turning constantly. Season with salt and rosemary.