Wash and clean the cucumber and slice or slice it thinly. Remove seeds from the melon and cut 100 g of flesh into balls with a ball cutter. Use the rest of the melon for other purposes.
Clean, wash and drain the watercress. Wash the mint, remove the leaves from the stalks and cut them into small pieces. Crumble feta cheese. Mix all ingredients in a bowl. Peel and finely dice the shallot.
Mix vinegar and lemon juice, season with salt, pepper and sugar. Fold in the oil and stir in the shallots. Pour vinaigrette over the salad.