Preheat the oven (electric cooker: 225 °C (circulating air: 200 °C/gas: level 4). Peel and finely dice onions. Clean, wash and cut leek into thin rings. Drain mushrooms well
Spread the Rösti unfrozen on a baking tray. Bake in a hot oven for about 12 minutes. Fry the mushrooms in the hot oil until they are very brown. Fry the onions and leek briefly
Stir in 1/4 l water, wine, cream and stock. Bring to the boil and simmer for about 4 minutes. Turn the Rösti and continue to bake for 8 minutes. Cut cucumber into slices and ham into strips
Stir the sauce thickener into the sauce, bring to the boil. Heat cucumbers and ham in it, season. Wash and chop parsley. Arrange everything, sprinkle with parsley