Mix lemon juice and approx. 3⁄4 l water. Wash, peel and quarter the quinces and remove the core. Dice the quinces and put them into the lemon water. Caramelise 2 tbsp. sugar until golden yellow. Stir in 200 ml apple juice, simmer while stirring until the caramel has dissolved.
Cut the vanilla pod lengthwise and scrape out the pulp. Drain the quinces. Add the quinces, vanilla pulp and vanilla pod to the apple stock and simmer for 15-20 minutes until the quinces are soft. Mix starch and 3 tbsp. apple juice, thicken compote with it.
Let it cool down. Remove the vanilla pod and cut into strips.
Crumble the pumpernickel. Stir yoghurt, crème fraîche and 3 tablespoons of sugar until smooth. Layer pumpernickel, yoghurt cream and compote alternately in glasses. Decorate with vanilla pod strips.