Trim outer leaves from cabbage. Quarter the heads, remove cores, and thinly shred using a sharp knife, mandoline, or food processor. Place the shredded green and red cabbage in a large mixing bowl.

Peel and julienne or grate the carrots. Add carrots to the bowl with the cabbage.

Thinly slice the red onion into half-moons, separate into rings, and add to the bowl.

Add chopped parsley to the vegetables and toss lightly to combine.

Tangy Dressing

Make dressing

In a medium bowl or a jar with a lid, combine mayonnaise, apple cider vinegar, yellow mustard, granulated sugar, and fresh lemon juice. Whisk or shake until smooth and sugar is mostly dissolved.

Season the dressing with salt, black pepper, and celery seed. Add hot sauce if using for a subtle heat. Whisk again to fully incorporate.

Taste the dressing. If you prefer a thinner, creamier slaw, whisk in 1–2 tablespoons of buttermilk or whole milk until it reaches desired consistency. Adjust salt, sugar, or vinegar to achieve a bright tangy-sweet balance.

Dress and rest

Pour about three-quarters of the dressing over the vegetables and toss thoroughly to coat. Reserve the remaining dressing in case you want to add more later. Cover the bowl and refrigerate for at least 30 minutes to allow flavors to meld and cabbage to soften slightly. For best texture and flavor, rest 1–2 hours or overnight, tossing once before serving and adding more dressing if needed.

Just before serving, taste and adjust seasoning with additional salt, pepper, sugar, or vinegar as desired. Serve chilled.