The fennel, garlic, and herb butter rubbed over the turkey are inspired by the flavors in porchetta, the classic Italian pork roast.
Arrange the rack in the lower part of the oven; preheat to 425°. Rinse the turkey. Using paper towels, pat the turkey dry all over, including the cavity.
In a medium skillet, cook the fennel seeds over medium heat just until warm and fragrant, about 2 minutes (do not toast; you just want to release the oils). Transfer to a cutting board and let cool. Using the bottom of a heavy skillet, coarsely crush the fennel seeds.
In a medium bowl, mix the fennel seeds, butter, lemon zest, grated garlic, chopped herbs, 2 tsp. pepper, and 1/2 tsp. salt.
Place the turkey in a large roasting pan fitted with a rack, breast-side up. Using your fingers and starting at the neck end, loosen the skin from the breast and thigh meat. Rub a little more than half of the herb butter (about 6 tbsp.) under the skin.
Rub the inside of the cavity with about 2 tbsp. of the remaining butter; season generously with salt and pepper. Stuff the cavity with the smashed garlic cloves, herb sprigs, and lemon wedges. Tuck the wing tips under. Using kitchen twine, tie the legs together loosely.
Rub the remaining 2 tbsp. butter on the outside of the turkey. Season generously with salt and pepper.
Transfer the turkey to the oven; reduce the temperature to 350°. Roast 1 hour. Rotate the pan. Continue roasting, tenting the turkey with foil if browning too quickly, until an instant-read thermometer inserted into the thickest part of a thigh registers 165º, 1 1/2 to 2 hours more.
Transfer the turkey to a cutting board and let rest for at least 30 minutes before carving.