Position an oven rack in the lowest position (remove the other racks); preheat to 350 degrees F. Remove the neck and giblets from the turkey and set aside for the gravy. Pat the turkey dry with paper towels and rub inside and out with 2 tablespoons each salt and pepper. Stuff the cavity with the onion, fennel, apple, thyme sprigs and lemon halves. Tie the legs together with kitchen twine. Put the turkey on a rack set in a large roasting pan and tuck the wings under the body. Let sit at room temperature while you make the lemon-thyme butter.
Melt the butter in a small saucepan over low heat; whisk in the lemon zest and chopped thyme. Let cool slightly, then brush all over the turkey. Transfer to the oven and roast 1 hour. Meanwhile, start your gravy (use 1/2 cup of your prosecco in place of the white wine in your gravy).
After the turkey has roasted 1 hour, pour the remaining prosecco all over the turkey (about 2 1/2 cups). Continue roasting, basting with the pan drippings every 30 minutes, until the skin is golden brown and a thermometer inserted into the thigh registers 165 degrees F, 1 1/2 to 2 more hours.
Transfer the turkey to a cutting board and let rest 30 minutes before carving; reserve the drippings for the gravy. Stir the herbs into the finished gravy.