Preheat the oven to 400 degrees . Rub the chicken all over with the butter and season with salt and pepper. Place skin side up in a roasting pan, surround with the onions and bake until golden brown and the juices run clear, about 45 minutes.
Toss the sliced fennel with the apple and olive oil; season with salt. Top with fennel fronds and set aside.
Transfer the chicken and onions to a platter. Place the roasting pan over medium heat and bring the pan drippings to a simmer. Stir in the brandy, then cook briefly to burn off some of the alcohol. Add the cream and cook, stirring, until thickened, about 5 minutes; season with salt and pepper. Pour the sauce over the chicken. Serve the salad alongside.