Start by adding your finely shredded cabbage and sliced green onions to a large heatproof mixing bowl. Toss them together gently with your hands or a spoon so they're evenly distributed before the dressing goes in.
In a small saucepan, whisk together the sugar, apple cider vinegar, celery seed, and salt over medium heat. Bring the mixture to a boil, then stir in the oil and let it boil again until the sugar fully dissolves.
Pour the hot dressing directly over your cabbage mixture and toss gently until every strand is coated. Cover the bowl and refrigerate for at least an hour so the flavors can fully settle in.
Once chilled, give the coleslaw a quick toss before serving since some liquid may settle at the bottom. If you'd rather skip the chilling step entirely, you can serve this dish warm right after making it.