How to Make It
In a small saucepan, bring the vinegar to a simmer. Put the serranos in a small heatproof bowl; pour the hot vinegar over them. Let cool, cover and refrigerate overnight.
In a skillet, heat the oil. Add the onion and cook over moderate heat until softened, about 7 minutes. Add the garlic and cook over moderately low heat, stirring, until golden, about 5 minutes. Add the caraway and stir in the kale. Cover and cook over moderate heat until the kale is sizzling, about 2 minutes. Uncover and cook, stirring, until the kale is just tender, about 2 minutes. Season with salt and pepper and toss with 3 tablespoons of the vinegar; reserve the remaining vinegar for another use. Serve the kale hot or warm.