1. MAKE THE STUFFING: In a large skillet, cook the pancetta over moderate heat, until softened, about 6 minutes. Add the onions and cook, stirring, until the pancetta and onions are browned, 6 to 7 minutes. Add the garlic, thyme and sage and cook for 1 minute. Stir in the bread crumbs and season with salt and pepper. Transfer to a bowl to cool.
2. MAKE THE TURKEY AND GRAVY: Preheat the oven to 400°. Set the turkey breast on a work surface, skin side down. Carefully remove the cartilage at the center; be careful not to cut through the skin. Fold the tenders outward, keeping them attached. Cover the turkey with plastic wrap and flatten it to an even 1 1/4-inch thickness with a meat pounder. Season the turkey with salt and pepper and evenly spread the stuffing over the meat. Beginning atone side, roll the turkey breast into a compact roast. Tie the roast in 5 places with kitchen string.
3. Line a 9-by-13-inch roasting pan with the pancetta slices. Scatter the onion, carrots, whole sage leaves and thyme sprigs in the pan and set the turkey on top. Rub 1 tablespoon of the butter over the turkey and season with salt and pepper.
4. Roast the turkey in the middle of the oven for 30 minutes. Add 1 cup of water to the pan and brush the turkey with the remaining 1 tablespoon of butter. Roast for 45 minutes longer, or until an instant-read thermometer inserted in the thickest part registers 140°; add more water to the pan if necessary. Transfer the turkey to a carving board.
5. Pour the juices from the roasting pan into a bowl. Spoon off the fat, reserving 2 tablespoons in a small bowl. Stir the flour into the fat to make a paste.
6. Set the roasting pan over moderately high heat. When it begins to sizzle, add the wine and cook until evaporated, scraping the pan. Transfer the contents of the pan to a large saucepan. Add the turkey stock and the pan juices and bring to a boil. Simmer until reduced to 3 cups, about 25 minutes. Strain the liquid, pressing hard on the solids, and return it to the saucepan. Bring to a simmer, whisk in the flour paste and cook for 5 minutes.
7. Meanwhile, in a small skillet, cook the chopped pancetta over moderate heat until browned, about 4 minutes. Add the chopped sage and cook until fragrant. Add the pancetta to the gravy and season with salt and pepper.
8. Discard the strings and carve the turkey into 1/3-inch slices. Arrange the slices on a platter and pass the gravy separately.
Make Ahead: The turkey and gravy can be prepared up to 4 hours ahead and kept at room temperature. Rewarm before serving.