How to Make It
Preheat the oven to 350°. Arrange the fennel wedges and onion slices in a roasting pan in an even layer. Pour the broth, wine and olive oil over the fennel. Season generously with salt and pepper and scatter the thyme sprigs on top. Cover with foil and bake for 1 hour, until the fennel is very tender.
Increase the oven temperature to 450°. Uncover the fennel and drizzle with balsamic vinegar. Cook on the top shelf of the oven for about 20 minutes, basting a few times with the cooking liquid, until the liquid has reduced and the fennel is glazed. Discard the thyme sprigs and serve the fennel hot or warm.