Place the butterflied turkey breast, skin side down, between two large sheets of plastic wrap, and pound to an even 12-millimeter thickness (the size of the turkey breast will be approximately 30.5 by 28 centimeters).
In a small bowl combine 1 tablespoon of the rosemary, the sage, lemon zest, garlic, ½ teaspoon salt, and ¼ teaspoon pepper. Sprinkle the mixture evenly over the turkey breast, and then roll up the turkey breast lengthwise to create a cylinder. Cover with the pancetta slices, overlapping them slightly, and then tie the cylinder with butcher’s twine at 5-centimeter intervals to secure the pancetta and maintain its shape. Brush the outside all over with the melted butter. Season evenly with 1 teaspoon of the rosemary, ¾ teaspoon salt, and ½ teaspoon pepper.
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Before you light the grill, place a large disposable foil pan underneath the cooking grate to catch the pancetta drippings. Prepare the grill for indirect cooking over medium heat (180° to 230°C).
Brush the cooking grates clean. Center the turkey over the foil pan and grill over indirect medium heat, with the lid closed, until an instant-read thermometer inserted into the thickest part of the breast registers 70° to 74°C, about 1 hour 20 minutes, turning two or three times. Meanwhile, make the apple relish.
In a medium skillet over medium heat on the stove, warm 1 tablespoon oil. Add the sage and swirl to release its fragrance, 20 to 30 seconds. Add the onion and cook until tender, about 5 minutes, stirring frequently. Add the apples and celery and cook for 2 minutes more, stirring occasionally. Add the water, lemon juice, honey, vinegar, ¼ teaspoon salt, and ⅛ teaspoon pepper. Bring to a simmer, cover, and cook until the apples are no longer crunchy but are still slightly firm to bite, about 3 minutes. Remove from the heat and cool to room temperature.
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After the turkey has cooked for about 40 minutes, place a perforated grill pan on the cooking grates over direct heat and preheat for 10 minutes. Put the potatoes in a medium bowl and toss with 2 teaspoons oil, the remaining 1 teaspoon rosemary, 1 teaspoon salt, and ½ teaspoon pepper. Spread the potatoes in a single layer on the grill pan and grill over direct heat until browned in spots and tender, 20 to 25 minutes, turning occasionally.
Transfer the turkey to a cutting board and let rest for 10 minutes. Remove the twine and cut the turkey crosswise into 12-millimeter slices. Serve warm with the potatoes and the relish.