Mix the quark and egg yolk. Peel the oranges so that the white skin is also removed. Cut out orange fillets between the parting skins. Collect the juice. Stir juice and sugar into the curd.
Whip cream and vanilla sugar until stiff and fold into the quark. Fill orange quark into dessert bowls. Decorate with filets and lemon balm. Add nut muesli or sprinkle over the quark.