Mix lemon juice, some light balsamic vinegar (if you don`t have one, you can leave it out) with a little salt and pepper and the sugar. Slowly stir in the cream (with a whisk or large fork), mix in the yogurt and fold in the chopped parsley. To taste. If necessary, add a little sugar, salt and lemon juice. If the dressing stands a little, it thickens. Then just stir in a little water.