Place potatoes in a large pot; add water to cover. Bring to a boil and cook until easily pierced with a fork, about 15 minutes. Drain. Let cool until easily handled, about 15 minutes. Peel and dice. Transfer to a large bowl.
Fill a saucepan 2/3 of the way with water and bring to a boil. Add cauliflower florets; cook until tender, 8 to 10 minutes. Drain in a colander and rinse with cold water to stop the cooking process, 1 to 2 minutes. Drain well. Blot with paper towels to remove excess moisture.
Place potatoes, cauliflower, hard-boiled eggs, green onions, celery, and black pepper in a large bowl; toss to combine.
Whisk eggs, stevia, and cornstarch together in a saucepan. Stir in evaporated milk, apple cider vinegar, white vinegar, and mustard. Cook over medium heat, stirring frequently, until dressing thickens, about 10 minutes. Remove from heat; stir in butter. Refrigerate until cool, about 30 minutes.
Fold mayonnaise into dressing. Stir into potato mixture gently until evenly coated.