Cook the pasta in boiling salted water for about 8 minutes. In the meantime, peel the grapefruits so that the white skin is completely removed. Cut out fillets between the parting skins and squeeze out the rest of the fruit.
Peel onions and cut them into fine rings. Wash the meat, dab dry and cut into strips. Heat oil. Fry the meat for about 3 minutes. Season with salt and pepper. Clean and wash the salad and pluck the leaves into bite-sized pieces.
For the sauce, stir yoghurt and 150 ml grapefruit juice until smooth. Season to taste with sugar and paprika. Arrange the salad ingredients with the sauce on plates.