Step 1: Place the turkey on a large cutting board. Carefully remove the skin in one piece from the breast meat using your fingers. You might need the aid of a knife; try not to puncture the skin. Lay the rough rectangle of skin flat on a rimmed baking sheet, inner side facing up. Arrange the whole sage and rosemary leaves and fennel fronds in a decorative pattern on the turkey skin.
Step 2: Using a boning knife and your fingers, debone the turkey breast by removing the wishbone and carefully following the curve of the ribs down toward the cartilaginous breast bone. Pull off the tenderloins, cut off and discard the sinews, and set aside. (You can save the rib cage for stock.)
Step 3: To give the breasts a more uniform thickness, butterfly the thicker ends of the breasts by making an angled cut toward the top; do not cut all the way through. Fold the flaps of meat outward. Position the reserved tenderloins lengthwise between the two breast halves.
Step 4: Make the seasoning paste: In the bowl of a food processor, combine the chopped sage, rosemary, fennel fronds, parsley, garlic, orange zest, fennel seeds, 2 teaspoons of salt, 1 teaspoon of black pepper, and the hot red pepper flakes. Process until the ingredients form a rough paste. Add 2 to 3 tablespoons of olive oil and pulse to make a spreadable paste.
Step 5: Using a rubber spatula, spread the paste over the turkey meat; be sure to coat the tenderloins as well. Carefully transfer the seasoned breast meat to the skin, positioning the breast parallel to and near the shorter edge.
Step 6: Using the skin as an aid, roll the turkey into a tight cylindrical shape. (The skin should remain on the outside of the roll but should completely enclose the meat.) Tie the roll, seam side down, at 1-inch intervals with butcher’s string. Fold the skin over the ends as if wrapping a gift: secure with a lengthwise-running piece of string. Lightly season the outside of the turketta with salt and pepper. Place a wire rack under the turketta on top of the baking sheet. Loosely cover the turketta with plastic wrap and refrigerate for 8 to 24 hours.
Step 7: When ready to cook, set up the grill for indirect grilling and preheat to 350 degrees. (If desired, you can also cook the turketta on a rotisserie.) Brush and oil the grill grate.
Step 8: Brush the turketta on all sides with melted butter. Place seam side down directly on the grill grate away from fire. Roast until the skin is crisp and golden brown and the internal temperature reaches 160 degrees on an instant-read meat thermometer, 1-1/2 to 2 hours. Brush with butter after the first hour. (Note: If using charcoal, you’ll likely have to replenish the fuel during the cook.)
Step 9: Transfer the turketta to a cutting board and let rest, uncovered, for 10 minutes. Remove the butcher’s string. Cut the turketta into 1/2-inch slices using a serrated knife or electric knife. Serve at once. Leftovers (in the unlikely case that you have them) make great sandwiches.