Prepare couscous in boiling salted water according to package instructions, drain and rinse with cold water. Drain. Clean and peel the cucumber and cut into small cubes. Wash tomatoes, quarter them, remove seeds and cut into small cubes.
Wash the lemon, grate dry and tear off the zest. Halve the lemon and squeeze. Wash the mint and dab dry. Pluck the leaves from the stalks and cut 3/4 into strips. Mix 2 tablespoons lemon juice, vinegar, 3 tablespoons water, 1 pinch sugar.
Season with salt and pepper. Fold in the oil. Stir in part of the mint strips. Mix the couscous and half of the vinaigrette in a large bowl. Add cucumber, tomatoes and the rest of the mint strips and season again with salt, pepper and cumin.
Decorate with lemon zests and mint leaves. Remaining vinaigrette extra is enough.