Prep the vegetables. Core the cabbage and slice it as thinly as you can with a sharp knife or a mandoline.
Aim for fine shreds so the dressing coats evenly. Grate the carrots and slice the red onion paper-thin.
Season the base. Add the cabbage, carrots, and onion to a large bowl. Sprinkle with 1 teaspoon kosher salt and toss.
Let it sit for 10 minutes to draw out a little moisture and soften the crunch.
Make the dressing. In a small bowl or jar, whisk together the apple cider vinegar, sugar, remaining 1/4 teaspoon salt, black pepper, celery seed, Dijon mustard, and oil until the sugar dissolves and the dressing looks glossy.
Taste and adjust. The dressing should be bright and slightly sweet with a savory backbone. Add a bit more sugar for balance or more vinegar for extra tang. Keep it punchier than you think—you’re seasoning a big bowl of veg.
Combine. Pour the dressing over the cabbage mixture and toss thoroughly.
Use your hands or tongs to lift and turn, making sure everything gets coated.
Rest. Let the slaw sit for at least 20–30 minutes before serving. This quick rest helps the flavors meld and the cabbage relax just enough while staying crisp.
Final seasoning. Taste again. Add a pinch more salt, a crack of pepper, or a splash of vinegar if needed.
If using, fold in parsley or a few jalapeño slices for a gentle kick.
Serve. Pile onto plates next to barbecue, top pulled pork sandwiches, or spoon over fish tacos. It’s also great with grilled vegetables or roasted potatoes.