How to Make It
Preheat the oven to 400°. Rub the butter all over the hen and inside the cavity. Season the skin and cavity with salt and pepper. Add 2 plum halves and 1 cup of the grapes to the cavity. Tie the legs together.
Set the hen in a medium roasting pan and roast for 15 minutes. Baste the bird, then roast for another 30 minutes. Scatter the remaining 6 plum halves and 1 cup of grapes around the hen and baste the bird with the pan juices. Continue to roast for about 20 minutes, basting 2 more times; the hen is done when an instant-read thermometer inserted in an inner thigh registers 160°.
Transfer the hen to a platter, cover loosely with foil and let rest for 10 minutes. Discard the string. Spoon the fruit from the cavity around the bird. Add the fruit from the pan and any juices to the platter, garnish with sage sprigs and serve. Pass the Chicken Liver Pâté with Toast separately.