Tess, tenderizing the kale is now a given for me. It makes all the difference in the world.
If I buy a bagged salad with chunks of fresh kale mixed in with softer greens, it never tastes as good. I prefer to cut my own rather than deal with precut kale. That way I get all the stems out and then crush by hand (with some dressing) to tenderize.
I also temper fresh onion, using a slightly simpler method: 2 cups of boiling water, 2 tsp of vinegar and soak the sliced red onion for 10 minutes. But this recipe sounds like it adds more to the overall taste.
Picking up a red onion now!