Combine kale, apple, celery, and onion in a bowl; set aside.
Cook bacon in a nonstick skillet over medium heat until crispy; transfer to a paper towel to drain.
Whisk lemon juice, vinegar, maple syrup, and mustard into the bacon fat until slightly thickened and creamy; remove from heat. Season with salt and pepper, to taste. Immediately pour over salad and toss to coat.
Crumble the bacon onto the salad. Serve immediately.