How to Make It
In a large skillet, heat the oil over moderately high heat until hot but not smoking. Reduce the heat to moderate, add the fennel and season with salt. Cook, stirring, until the fennel browns lightly and absorbs the oil, about 5 minutes. Add the stock, cover and cook over moderately low heat until the fennel is meltingly tender throughout, about 30 minutes. If any stock remains in the skillet, remove the fennel and reduce the liquid over high heat until just a few tablespoons of thick sauce remain. If the fennel was removed, return it to the skillet. Cover and set aside.
Preheat the broiler. Arrange the fennel, slightly overlapping in a large gratin dish. Sprinkle the cheese evenly over the top. Broil about 3 inches from the heat for 3 to 5 minutes, or until the cheese melts and the fennel sizzles. Serve hot.