How to Make It
Preheat the oven to 350°. Butter a 9-by-13-inch glass or ceramic baking dish. Put the fennel bulbs in a large saucepan of water and add 1 teaspoon of salt. Bring the water to a simmer over moderate heat and cook the fennel for 1 minute. Drain the fennel and let cool completely; pat dry with paper towels. Cut the fennel lengthwise through the core into 1/4-inch-thick slices.
In a large skillet, heat the olive oil. Add the onions and cook over moderately high heat, stirring occasionally, until golden and softened, about 10 minutes. Season with salt and pepper.
Spread one-third of the onions in the baking dish and arrange half of the fennel slices on top. Season with salt and pepper and sprinkle with 1/2 cup of the Pecorino. Top with half of the onions, the remaining fennel and then the remaining onions. Sprinkle the remaining 1/2 cup of Pecorino and the breadcrumbs on top and dot with the butter. Cover with foil and bake for 1 hour, or until the fennel is tender.
Preheat the broiler. Uncover the fennel and broil for 2 minutes, rotating the dish, until evenly browned. Let the fennel stand for 5 minutes before serving.