Poach chicken in boiling, salted water.
When it is cool enough to handle, remove any bones, skin and visible fat. Cut chicken into 1/2-inch cubes.
Put chicken and celery in a large bowl and sprinkle with vinaigrette.
Chill for 1 hour or more.
In a separate bowl, mix mayonnaise, mustard, salt, pepper and celery seed.
Pour dressing over chicken and celery, and toss gently but thoroughly.
Refrigerate, covered, for up to 8 Hours.
When ready to serve, line a serving bowl or platter with lettuce leaves. Turn the chicken salad out onto the leaves and garnish with tomato quarters.
