Use tongs to remove the browned chicken from the pan, and set aside for now.
Add the sliced onion and fennel to the pan. Saute for a few minutes in the rendered chicken fat, until they are slightly softened. Season with salt and pepper.
Pour in the wine, stirring up any of the fond that may be stuck to the bottom of the pan. Beware of splatter.
Add the chicken back to the pan, skin side up, layering over the onion and fennel. Place the skillet in the oven and bake for 15-20 minutes, until the thighs are cooked through.
Remove from the oven and allow the chicken to rest for about 5 minutes, then spoon the fennel frond sauce over top.