Boil chicken breasts in a pot with broth and cook over low heat for about 20 minutes. Remove chicken from pot and allow to cool. Cut chicken into small cubes.
Rinse celery, trim and cut diagonally into thin slices. Set aside celery leaves. Rinse grapes, trim, cut in half, cover and let rest in a cool place. Rinse lettuce, trim and spin dry. Mix mayonnaise in a bowl with yogurt, mustard, sugar, lemon juice and lemon zest and season to taste with salt and pepper.
Mix chicken in a bowl with celery and mayonnaise mixture. Add grapes and mix. Add lettuce to the bowl and serve garnished with celery leaves.