Use a mandolin, food processor, or sharp knife to shred the cabbage. Alternatively, purchase a 1 lb bag of packaged shredded cabbage coleslaw mix.

Add the shredded cabbage, carrots, and onions to a large bowl or colander and sprinkle with half of the salt. Let it sit for about 30 minutes to strain excess liquid.

Prepare the dressing by whisking together the apple cider vinegar, Dijon mustard, olive oil, brown sugar, celery seed, garlic powder, and the remaining kosher salt in a small bowl. Adjust seasonings as needed.

Once the cabbage mixture has been drained, transfer it to a large mixing bowl. Add the dressing and green onions, and mix well.

Let the coleslaw marinate in the fridge for at least 30 minutes before serving. Toss, taste, and adjust seasoning if necessary before serving.