Salting and draining the cabbage prevents that pool of liquid that can collect in the bottom of the bowl. Be sure to taste before adding salt to the finished slaw — even after rinsing and draining, the cabbage remains slightly salty. You can shred (rather than chop) the cabbage and carrots if you prefer a finer slaw.
Toss the cabbage with the salt and place in a colander over the sink. Let drain for 1 hour. Then rinse the cabbage with cold water, drain it, squeeze gently to remove moisture and spread on kitchen towels to dry for about 15 minutes.
While the cabbage drains, put the oil, vinegar and garlic powder in a small bowl and stir to combine. Lightly crush the cumin seeds in your palm and add to the mixture, then stir in the chopped serrano pepper.
Put the drained-and-dried cabbage in a large bowl. Stir in the carrots and onions, then the vinegar-based dressing. Taste, then add salt if needed. Sprinkle chives or parsley on top and serve.