For the Slaw: In a large colander, combine cabbage, carrot, and apple. Sprinkle with sugar and salt and toss to combine. Let sit for 5 minutes, then rinse thoroughly under cold running water.
Meanwhile, prepare the dressing: In a small bowl, whisk sour cream, mayonnaise, apple cider vinegar, sugar, honey, black pepper, mustard powder, and poppy seeds to combine.
To Assemble: Transfer vegetables to a salad spinner and spin dry. Alternatively, transfer to a 13- by 18-inch rimmed baking sheet lined with a triple layer of paper towels or a clean kitchen towel. Blot mixture dry with paper towels, and place vegetables in a large bowl.
Pour dressing over slaw and toss to coat. Adjust seasoning with additional salt, pepper, and sugar as needed.
