Make the Poppy Seed Dressing: In a small bowl, whisk together mayonnaise, vinegar, syrup and sugar until smooth. Whisk in the poppy seeds and salt until combined. Refrigerate until ready to serve. (You can also use a mason jar or salad dressing cruet, shaking vigorously instead of whisking.)
Assemble the Salad: In a large bowl, toss together the kale, shaved Brussels sprouts, cranberries, sunflower seeds and apple. Drizzle with dressing and toss to combine. (Since the kale and Brussels sprouts are hearty, you can prepare the salad a few hours ahead and add the fresh apple right before serving.)