Whisk together the maple syrup, mustard, garlic, vinegar, olive oil, salt, and pepper in a large bowl until well blended.
Add the red onion, kale, carrots, cabbage, and apple to the dressing and mix until everything is coated.
Sprinkle the sunflower seeds or nuts on top of the slaw and mix until everything is evenly distributed.
Refrigerate for at least 30 minutes before serving.