Special equipment: a roasting pan with a v-rack and butcher's twine
Remove the turkey from the refrigerator and let come to room temperature for 1 hour before roasting.
Adjust an oven rack to the lowest level and preheat the oven to 350 degrees F.
Add the sage, olive oil, rosemary, fennel seeds and garlic to a mortar and pestle and mash until a paste forms. Season the mixture with a big pinch of salt and pepper.
Pat the turkey dry with paper towels, place in a roasting pan with a v-rack and season the turkey's cavity with salt and pepper. Separate the skin from the breast meat carefully with your hands, being mindful not to tear the skin. Massage the herb-fennel mixture into the meat under the skin. Press the skin down over the herb mixture and season the entire bird generously with salt and pepper.
Loosely stuff the turkey with the stuffing. (Place any remaining stuffing into a buttered 2-quart casserole dish and bake separately for 35 minutes.) Layer the pancetta over the turkey, overlapping slightly. Tie the legs together with butcher's twine and tuck the wings under. Pour the beer into the bottom of the roasting pan.
Roast the turkey, basting with the beer and pan juices every 30 minutes, for 2 hours, then cover loosely with foil to keep the pancetta from getting too dark. Continue to roast until an instant-read thermometer reads 170 degrees F when inserted into the thickest part of the thigh and the turkey is a lovely golden brown, another 1 hour and 45 minutes (3 hours and 45 minutes total). Allow the turkey to rest for 30 minutes before carving.