Shred everything thin. Thinly shred the green and red cabbage (a mandoline or the slicing disc on a food processor makes quick work of it), and grate the carrots. Thin shreds soak up dressing better and give you that classic deli texture.

Salt and wilt — the key step. Toss the shredded cabbage with the 1 teaspoon of salt in a colander set over a bowl or in the sink. Let it sit for 30 minutes. You'll see liquid start to bead up and drain out — that's the water that would've wrecked your slaw.

Squeeze it dry. Give the cabbage a quick rinse to wash off the excess salt, then squeeze it hard — a big handful at a time, or wrap it in a clean kitchen towel and wring it out. Get it as dry as you can. This is the single step that keeps the finished slaw from going watery.

Whisk the dressing. In a large bowl, whisk together the mayonnaise, apple cider vinegar, Dijon, sugar, celery seed, salt, and pepper until smooth. Add the buttermilk if you'd like a slightly looser, more pourable dressing.

Toss it together. Add the squeezed cabbage, grated carrots, and green onions to the bowl and toss until everything is evenly coated. It'll look like the right amount of dressing now and stay that way, because the cabbage isn't going to release more water.

Chill and serve. Cover and refrigerate for at least 1 hour so the flavors meld and the celery seed blooms. Give it a quick toss before serving. Taste and add a pinch more salt or a splash of vinegar if it needs brightening.