Heat the oil in a large skillet over high heat. Add the onions, salt, pepper, and red pepper flakes and stir-fry for 2 minutes. Add the garlic, kale, and stock and cook, stirring occasionally, for 8 to 10 minutes; add a splash of cider vinegar in the last minute of cooking. Remove from the heat. Serve immediately.
Basic Chicken Stock:
In a large stock pot, over high heat, add the oil. When the oil is hot, add the onions, carrots, and celery. Saute for 2 to 3 minutes. Add the remaining ingredients and bring to a boil. Reduce the heat to low and simmer for about 2 hours. Remove the stock from the heat and skim off any scum that is on the surface. Strain the stock through a large fine-mesh sieve. Discard the bones and vegetables.