How to Make It
In a large skillet over medium heat, cook the bacon until crisp, 6 to 8 minutes. Transfer to a paper towel-lined plate. Let cool, then crumble.
Add the onion and apple to the drippings in the skillet and cook until tender, 4 to 6 minutes. Add the kale, season with ¾ teaspoon salt and ¼ teaspoon pepper, and cook, tossing occasionally, until tender, 8 to 10 minutes more. Mix in the bacon and vinegar.