1. Combine first 3 ingredients in a small bowl, and let stand at least 10 minutes to soften. Whisk in mustard and olive oil.
2. Combine spinach, arugula, and celery leaves in a large bowl. Add apple and cheese. Dress salad with vinegar mixture, toss, and serve immediately.
*For testing purposes, we used Grey Poupon brand Country Dijon Mustard.