Place potatoes in a small saucepan; add water to cover. Bring to a boil. Reduce heat; cook, uncovered, 10-15 minutes or until tender. Drain; cool completely., In a small bowl, whisk onion, mustard, vinegar, lemon juice and seasonings. Gradually whisk in oil until blended. , In a large bowl, combine salad mix, chicken, celery and potatoes. Just before serving, add dressing; toss to coat. If desired, serve on small romaine leaves. Top with bacon.