How to Make It
Preheat the oven to 425°. Brush the onion halves with olive oil, season with salt and arrange cut side down in an ovenproof medium skillet. Add the chicken stock and scatter the bay leaves around the onions. Cover tightly with foil and bake for about 1 1/2 hours, until the onions are very tender.
Meanwhile, in a small skillet, toast the fennel seeds over moderate heat until fragrant, about 3 minutes. Transfer to a work surface and let cool, then coarsely crush the seeds. Transfer to a small bowl, add the panko, sage and the 2 tablespoons of olive oil and toss. Season with salt.
Carefully turn the onions cut side up in the skillet. Spoon the fennel breadcrumbs on top and bake for about 15 minutes longer, until the crumbs are lightly browned and crisp. Discard the bay leaves and serve the onions hot or warm.