In a bowl, whisk the quark with the yogurt and sugar until thoroughly combined. In another bowl, mash the drained mandarin oranges with a fork. Stir in the jam. Stir the orange mixture into the quark.
Peel the oranges and grapefruit and use a sharp knife to cut into segments. Collect any excess juice. Place the citrus in the bottoms of jars or tall dessert glasses. Drizzle the juice on top. Place the quark mixture on top and serve.