Bring 6 cups of water to the boil, remove from the heat and stir in the puree with a whisk. Leave to swell for 1 minute. Add cream and stir again. Heat the sauerkraut in a pot. Wash the chives, cut into fine rolls and fold into the cabbage, except for 1 tablespoon. Spread the sauerkraut on the ham slices and roll up. Put the mashed potatoes into an ovenproof dish.
Place ham and sauerkraut rolls on top. Grate cheese and sprinkle over the rolls. Bake in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 10 minutes. Peel onion and cut into rings. Heat the oil in a pan. Fry the onion rings and spread them on the casserole with the remaining chives.