For the couscous salad, place the couscous in a bowl and pour boiling soup over it. Allow to swell for approx. 5 min. and loosen with a fork. Chop the onion.
Peel cucumber and cut into small pieces. Dice tomatoes and cut feta into small cubes. Mix olive oil with lime juice, salt, pepper and sugar.
Mix couscous with remaining ingredients and marinade and let stand for a good half hour.