Pickle onions by mixing red wine vinegar, water, and sugar in a small bowl and stirring to dissolve. Add sliced onions and let set in the fridge while prepping the rest of the salad. The liquid should cover the onion, so add a bit more vinegar and/or water if needed.
Add kale to a large mixing bowl with 1-2 tsp (amount as original recipe is written // adjust if altering batch size) olive oil and squeeze over reserved grapefruit or orange juice. Use your fingers to massage the kale to break down the bitterness and stiffness.
Next add apple, grapefruit or orange segments, and pomegranate or cranberries and set aside.
Prepare dressing in a small mixing bowl by whisking vinegar, mustard, sweetener, and salt and pepper. Stream in olive oil while mixing, then taste and adjust seasonings as needed. I added a splash of orange juice to brighten up the flavors, but this is optional.
Add pickled onions to the salad, as well as the dressing (or reserve it for serving), toss and serve. Leftovers keep up to a couple of days in the fridge, though best when fresh.
* Find more tips for pickling onions at The Kitchn. * Nutrition information is a rough estimate calculated with dressing.