Bring the milk to the boil, stir in the oatmeal and cook over low heat for about 5 minutes. Then remove from the heat and transfer to a bowl, allow to cool.
Fillet the grapefruit, catching the dripping juice. Dice half of the fillets. Mix the quark and pear syrup, then also the diced grapefruit fillets, into the oat cream. Stir in enough drained grapefruit juice until the mixture is creamy.
Transfer to a bowl and garnish with the remaining grapefruit fillets.