In the same skillet over medium-high heat, brown the radicchio on all sides in 45 ml (3 tablespoons) of the reserved duck fat. Season with salt and pepper. Add the apples and continue cooking for about 2 minutes. Add the broth and maple syrup. Bring to a boil. Place the duck breasts on top, skin side up, and transfer to the oven. Roast until medium-rare, about 7 minutes. Transfer the breasts and radicchio to a warm plate, cover loosely with aluminum foil and let rest 5 minutes.