Kale may well be the most humble of greens, eschewed by many who perceive it to be bitter, but this preparation is sure to convert those who doubt its charms. The earthy, rich flavor and aroma of truffle oil elevate this rustic green to new heights. Add a layer of nutty Gruyère cheese and a crisp topping of panko bread crumbs, and you have a vegetable dish that will have everyone begging for seconds.
Position a rack in the middle of the oven and preheat the oven to 350 degrees F.
Heat 2 tablespoons of the olive oil in a large skillet over medium heat. When the oil is hot, add the onion and cook, stirring frequently, until softened, about 8 minutes. Add the garlic and cook, stirring constantly, for 2 minutes. Add the kale and chicken broth and cook, covered, until the kale is tender, 10 to 20 minutes (depending on the variety of kale).
Meanwhile, heat the remaining olive oil in another skillet and stir in the panko. Cook over moderate heat until the panko is lightly golden, about 5 minutes, stirring often and watching closely so that the crumbs don't darken or burn. Set aside.
When the kale is tender, remove the pan from the heat and stir in the tarragon and truffle oil. Transfer the mixture to a 1-quart ceramic or glass baking dish. Sprinkle the cheese on top of the kale and then top with the panko. Bake until the crumbs are golden and the cheese has melted, 20 to 25 minutes. Serve hot.