Add the sliced bacon to a large skillet over medium heat. Once the fat begins to render from the bacon, add the red onion. Continue to cook, stirring occasionally, until the bacon is crispy and the onion is soft, about 10 minutes. Drain any excess fat from the pan.
While the onions cook, whisk together the dressing ingredients in a large serving bowl until they are smooth and creamy.
Add the kale to the serving bowl and toss to coat. Once the bacon and onions have finished cooking, let them cool for 1 minute. Then, add them to the bowl with the kale.
Add the walnuts, sliced apple, and blue cheese to the salad and toss everything together to combine. Serve immediately.
Notes
You can use pre-packaged baby kale to save on prep time.To toast the walnuts: Preheat your oven to 400 degrees F and place the walnuts on a baking sheet in an even layer. Toast them for 6-8 minutes. Keep a close eye on things as the nuts can burn easily. (I do this while the bacon and onions are cooking, so everything finishes together.)