Remove leaves and core of cauliflower. Break florets into bite-size pieces, and cut stem into bite-size pieces as well, if desired. Total yield should be 4 cups of cauliflower pieces. Fill a medium saucepan 2/3 full of water and bring to a boil. Add cauliflower and cook 8–10 minutes until soft but not mushy. Drain cauliflower in a colander and rinse under cold water for 1–2 minutes to stop the cooking process. Drain well. Blot pieces with paper towel to remove excess moisture. In a large bowl, combine cauliflower, egg, red onion, and celery. In a small bowl, whisk together mayonnaise, pickle relish, mustard, celery seed, and black pepper. Pour mayonnaise mixture over vegetables and mix well. Cover and chill 2 hours prior to serving.