Put the chicken breasts in a large pot of boiling salted water. Reduce the heat so the water just simmers and cook for 10 to 15 minutes or until done. The chicken should be white all the way through when pierced with a thin-bladed knife. Remove from the water and let cool.
Meanwhile, in a large bowl combine the onion, celery, lemon juice, yogurt, tarragon, mustard, salt and pepper; mix well.
When the chicken is cool enough to handle, cut into ½-inch cubes and add to the bowl along with the grapes. Stir to combine and serve over the greens.
