For the dressing:
Place the vinegar, honey, mustard, poppy seeds, and salt in a large, nonreactive bowl, season with pepper, and whisk to combine. While whisking constantly, slowly add the oil until all of it is incorporated. Add the onion, stir to combine, and set aside.
For the coleslaw:
Wash and dry the kale. Cut out and discard the tough stems. Arrange the leaves into stacks, slice crosswise into 1/4-inch ribbons, and add to the bowl with the dressing. Core the apples, cut them into 1-1/2-inch-long matchsticks, and add to the bowl. Toss to combine. Let the coleslaw sit for at least 15 minutes at room temperature and up to 1 day in the refrigerator for the flavors to meld. Toss again before serving.
Adapted from Chow.com
Add-onsCondiments & Sauces