Place the turkey breast between two sheets of plastic wrap and pound lightly with a meat mallet to an even thickness of about 3/4 inch. Set aside while you prepare the stuffing.
Toast the pecans in a small dry skillet over medium-high heat until fragrant, stirring frequently, about 3 minutes; set aside. Preheat the oven to 375°F.
Place the cranberries in a small saucepan and cover with water. Bring to a boil, cook for 1 minute, then remove from the heat, drain, and set aside.
Heat 1 tablespoon of the oil in a large heavy skillet over medium heat. Add the diced onion and cook, stirring, until golden but not blackened, 12 to 15 minutes. Add the garlic and cook another 2 to 3 minutes. Add the bread, cranberries, pecans, 2 tablespoons of the fresh or 2 teaspoons of the dried sage, and 1/2 to 3/4 cup of the broth, depending upon the consistency of the stuffing (you want the mixture to be moistened, but not too wet, since the turkey will release moisture when cooked). Cook over low heat for 2 to 3 minutes, then remove from the heat.
Sprinkle the salt and pepper over both sides of the turkey breast. Spread the stuffing over one side of the turkey, leaving about 1-1/2 to 2 inches uncovered on all sides. Roll up and secure tightly with kitchen twine, trying to keep all the stuffing intact.
Heat the remaining 1 tablespoon oil in a large Dutch oven or medium roasting pan over medium heat until hot. Sear the stuffed turkey breast on all sides until lightly browned, 3 to 4 minutes per side. Sprinkle the sliced onion around the turkey, pour in 1-1/2 cups of the broth, cover tightly and roast in the oven until an instant-read meat thermometer inserted in the thickest part registers 165°F, 60 to 65 minutes. Remove the turkey breast from the oven, transfer to a cutting board, tent with aluminum foil, and let rest while you make the gravy.
Add the cider, 1/2 cup of the broth, the vinegar, and the remaining 2 teaspoons fresh or 1 teaspoon dried sage to the roasting pan, bring to a boil, and cook, stirring occasionally, until the liquid is reduced by one-third, about 10 minutes. Slowly add the cornstarch mixture, stirring constantly, and cook for 3 minutes more. The gravy should not be thick, just slightly thicker than a jus. Season with salt and pepper to taste.
Remove the twine from the turkey breast and cut into 1-1/2-inch-thick slices. Serve with the gravy on the side in a gravy boat.