Peel the grapefruit so that the white skin is completely removed. Cut the grapefruits into slices and arrange them on 4 plates. Clean lamb's lettuce, wash thoroughly and drain. Chop the walnuts, dice the gorgonzola. Mix white wine vinegar, salt, pepper and sugar. Whip oil into it and finally stir in walnuts and cheese. Arrange the lamb's lettuce on the grapefruit slices and spread the walnut vinaigrette on top