For the porchetta: The day before roasting, chop 1 teaspoon of the fennel seed and mix it with the orange zest and 2 1/4 teaspoons of the salt. Rub the turkey breast with the salt mixture, then transfer it to a gallon-size freezer bag and refrigerate over night.
The day of roasting: Preheat the oven to 400 degrees F. Remove the turkey breast from the refrigerator and allow it to come to room temperature for 20 minutes.
Meanwhile, toast the panko in a 12-inch ovenproof skillet over medium heat, stirring with a rubber spatula until golden brown. Remove to a medium bowl and add the remaining 1 teaspoon fennel seed, the red pepper flakes and 1/2 teaspoon of the salt. Set aside.
To the same pan, over medium-high heat, add 2 tablespoons of the olive oil, then the apples, shallots, fennel, rosemary and the remaining 1/2 teaspoon salt. Cook until the fennel softens, about 5 minutes; add to the bowl with the panko and toss to combine.
Unfold the butterflied turkey breast and spread the stuffing evenly over the surface of the breast. Tightly roll up the breast and tie it with butcher's twine at 2-inch intervals, then tie it once lengthwise.
Place the skillet over medium-high heat and add the remaining 2 tablespoons olive oil. Sear the turkey on all sides until golden brown, about 3 minutes per side. Transfer to the oven and roast until an instant read thermometer reads 155 degrees F at the thickest point, about 40 minutes. Remove the turkey to a cutting board, tent with foil and rest for 15 minute before slicing.
For the gravy: While the porchetta is resting, prepare the gravy. Over medium-high heat, add the rosemary sprigs and flour to the skillet the turkey was cooked in; whisk to form a smooth paste. (If there aren't enough pan drippings to make a paste, add a tablespoon of olive oil.) Cook the paste to a medium amber color over medium-high heat, 3 to 4 minutes. Add the white wine and cook, whisking, for 1 minute more. Add the chicken broth, salt and pepper and whisk until smooth. Bring the gravy to a boil, lower the heat to medium and simmer for 5 minutes. Strain the gravy just before serving. Serve alongside the porchetta.