Bring 300 ml water, cassis, 2 tablespoons sugar and 4 tablespoons lemon juice to the boil in a high pot. Peel the pears, cover and stew for 15-20 minutes until the pears are soft.
In the meantime, chop the walnuts. Caramelise 2 tablespoons of sugar in a pan until golden brown. Immediately turn the nuts in it and spread them quickly on a piece of baking paper. Allow to set.
Lift the pears out of the stock and let them drain. Boil down the open stock for 10-15 minutes until thick. Mix crème fraîche and 1 tablespoon lemon juice. Whip the cream until stiff, adding 1 tbsp. sugar and vanilla sugar.
Fold into the crème fraîche, spread over four dessert plates. Arrange the pears on the cream. Drizzle syrup over the pears. Crumble the brittle and sprinkle over it.