In a medium bowl, make the dressing by whisking together the apple cider vinegar, lime juice, maple syrup or honey, olive oil, salt, and pepper. Add the thinly sliced red onion and set it aside to soak for 10 to 15 minutes.
Meanwhile, place the chopped kale, chopped apples, and shredded carrots in a large bowl.
Add the red onion and dressing to the large bowl, and stir to combine evenly.
Taste the slaw and add additional oil, vinegar, or honey as desired. The onions soak up some of the dressing, so you might want a bit more.
Cover the slaw and let it sit for at least 30 minutes (an hour is even better) to let the flavors come together. If you aren’t serving this within an hour of making it, refrigerate it until ready to serve. This slaw presents best on the day it’s made, but leftovers can be eaten the next day, too.