Cut melon into slices, remove skin. Cut the flesh into large cubes. Wash limes, grate dry, cut in half and squeeze juice. Sprinkle melon with lime juice. Lightly roast the walnuts in a pan without fat. Remove from the pan, let cool and chop coarsely.
Wash the mint, shake dry and pluck the leaves from the stalks. Arrange watermelon in bowls. Coarsely crumble cheese over it. Sprinkle with nuts, mint and a little pepper and sprinkle with oil. Garnish with squeezed lime halves if necessary.