1 Preheat the oven to 200°C (approximately 400°F). Line a deep baking sheet with parchment paper.
2 For the topping: Whisk the sour cream, lemon juice, eggs and sugar together in a bowl.
3 For the cake: Cream the butter, sugar and vanilla sugar until fluffy. Gradually stir in the eggs. Combine the flour, oats, baking powder and salt then add to the batter alternately with the milk. Spread over the prepared baking sheet.
4 Peel the apples and pears, cut into quarters and core. Rinse the plums, cut in half and pit. Cut the apples, pears and plums into thick wedges. Arrange in 5 cm (approximately 2-inch) wide strips down the batter, separating the fruit, then sprinkle the walnuts between the fruit strips.
5 Bake for about 35-40 minutes, until golden brown. Let cool on the baking sheet.