Put the bacon in a small stock pot. Turn the heat to medium and cook, stirring it occasionally, until crisp and browned but not dry, about 7 minutes. While the bacon cooks, halve and thinly slice the red onion.
Add the onion to the bacon and the drippings. Cook, stirring and scraping the browned bits from the bottom of the pot, until softened, about 4 minutes. Add the beans, their liquid and a half can of water to the pot, along with a generous grind of black pepper, and bring to a boil. Cover, reduce the heat to low, and simmer 15 minutes to bring all the flavors together.
While the beans simmer, remove the thick stems from the kale and cut or tear into bite-size pieces. Stir and fold it into the beans. Then cover and cook until the kale is as crisp or tender as you like, 5 to 10 more minutes. Taste, then add more black pepper, if desired. Remove from the heat, and stir in 1 tablespoon of the cider vinegar and serve immediately. Pass around the cider vinegar to drizzle on the finished dish.
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