To make the vinaigrette: Add the onion to a food processor ( I use a mini food processor) with the Dijon mustard, and sugar. Blend until the onion is finely chopped. Add the vinegar and blend. Slowly add the olive oil until blended. Add the celery seed and blend in. Add salt to taste if desired.
To assemble the salad: Wash and dry the lettuce. Tear into bite-size pieces. Place lettuce on plates top with the chicken, slices of pear, blue cheese, and pecans. Drizzle the salad dressing over the top of the salad. Serve.