Place the cabbage and carrots in a large bowl and toss thoroughly.: Right away you will notice the crisp sound as the shredded green cabbage , red cabbage , and grated carrots hit the bowl, a bright, dry aroma that signals freshness. Use your hands or tongs to lift and fold rather than beating the vegetables down, which helps maintain individual shreds and prevents bruising. Tossing distributes the vegetables evenly so every forkful will have color and crunch. A common error is packing the bowl too tight while tossing, which can crush the cabbage and make the salad limp; leave room to move. As you toss, look for even dispersion of orange carrot strands among the layers of purple and pale green, and listen for the steady, crunchy swish telling you the pieces are separated well.
Combine the remaining ingredients in a medium bowl and mix well.: When you whisk together Miracle Whip , sugar , grated horseradish , dry mustard , white pepper , kosher salt , garlic powder , and celery seed , the dressing becomes creamy and slightly glossy, releasing a tangy perfume of mustard and horseradish. Whisk briskly so the sugar dissolves and the horseradish disperses evenly; you want a smooth coat rather than grainy pockets of sugar. The technique matters because a well emulsified dressing clings to the shreds, creating consistent flavor in every bite. Avoid under mixing, which can leave concentrated spots of spice, and over mixing, which will warm the dressing slightly; work until the mixture is homogeneous and cool to the touch. Taste the dressing before combining it with the vegetables to check balance, adjusting only if something is noticeably missing.
Add the dressing to the slaw and toss until thoroughly coated.: As soon as the dressing meets the shredded vegetables, you will feel the texture shift from dry to velvety, and the visual tone will change as the strands take on a soft sheen. Use a gentle folding motion to ensure that each shred is lightly coated, turning the bowl as you go so the dressing reaches the bottom. This technique matters because aggressive stirring can crush delicate shreds and squeeze out moisture, while gentle folding preserves the crispness. One pitfall is adding all the dressing at once and over saturating the bottom layers; instead, drizzle and fold in batches if you are concerned about even coverage. Aim for a uniform, slightly glossy appearance without any large pools of dressing.
Refrigerate for at least 4 hours to allow the flavors to develop.: When you cover and chill the slaw, the real transformation begins, as the dressing softens the cabbage enough to meld the flavors while keeping a pleasant crunch. Cold temperature also calms the sharpness of horseradish and white pepper, letting sweetness and savory notes harmonize. I recommend a minimum of 4 hours, but overnight often improves depth; schedule this step when you can plan ahead. A common mistake is serving too soon, which results in a disjointed flavor profile and a firmer, less integrated texture. After chilling, give the slaw a gentle toss to redistribute any settled dressing and taste for final seasoning before serving.