In a large bowl, combine the lettuce, celery, onion, grapes, and blue cheese.
In a small bowl, whisk together the lemon juice, vinegar, oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Divide the salad and chicken among individual plates. Serve with the vinaigrette.
Tip: Omit the chicken and this salad becomes a delicious side dish for grilled steak, pork chops, or burgers.