For the brine: Combine the water, kosher salt, honey, bay leaves, garlic, black peppercorns, rosemary, thyme and parsley in a large pot over high heat. Bring to a boil, then transfer to an airtight container and refrigerate until completely cooled.
For the turkey: Invert the turkey breast on a cutting board so the cavity is facing upward and the neck end is facing away from you. Starting at the neck end, use a very sharp boning knife to gently slice the meat away from the bone on one side of the breast, then slice away on the other side. Cut away constricting pieces of skin at the top and bottom of the breast as needed, making sure to keep the skin intact as much as possible. When most of the meat on both sides has been cut from the bone, use short strokes to detach the remaining meat and skin from the backbone on both sides, being careful not to cut all the way through the skin. Once the carcass is fully detached, you should have a large flap of skin with the 2 breasts connected to it (butterflied).
Place the deboned turkey in a large brining bag or large resealable plastic food storage bag and add the cooled brining liquid. Seal and place inside a large bowl or pan; refrigerate, allowing 1 hour for every pound of turkey (be sure to calculate using the deboned weight of the bird).
When ready to roast, preheat the oven to 325 degrees. Have ready a large roasting pan fitted with a flat rack inside.
Rinse the turkey breast well and pat dry. Season the skin lightly with salt and pepper. Discard the brining liquid.
Open the turkey breast and lay it on a clean work surface, skin side down. Spoon 2 cups of the cooled dressing down the center and fold the turkey breast over. Using a 5-foot length of kitchen twine, tie a loop around one end of the turkey, securing it with a square knot. Spiral the twine around and under the turkey breast at 1-inch intervals all the way across; tie at the other end, then tie diagonally from one end to the other. The turkey breast should look compact and neat but not tied too tightly.
Place the rolled turkey breast on the roasting rack; lightly grease the skin all over with nonstick cooking oil spray. Roast for about 20 minutes per pound or until a meat thermometer inserted into the center of one side of the breast reaches 160 degrees; the skin should be golden brown (the final temperature of 165 degrees will be reached during the post-roast resting period). Transfer the pan to the counter and tent the turkey breast loosely with aluminum foil for at least 20 minutes before cutting. Pour the pan juices into a fat separator measuring cup.
For the sauce: Heat the oil in a medium saucepan over medium-low heat. Add the shallots and garlic and cook for 2 to 3 minutes, stirring occasionally. Add the white wine and increase the heat to medium-high; let it come to a boil, then reduce the heat to medium-low and cook, uncovered, for 15 to 20 minutes or until the liquid has reduced to about 1/3 cup.
Add 3 cups of the chicken broth and increase the heat to medium-high; bring to a boil, then reduce to medium or medium-low and cook for about 25 minutes or until the liquid has reduced to about 2 cups. (At this point, the sauce can be cooled to room temperature, then covered and refrigerated for up to 3 days.).
When the sauce has been reduced, whisk together the cornstarch and the remaining 3 tablespoons of (unheated) chicken broth in a small bowl to form a slurry, or thickener, for the pan sauce. Whisk the slurry into the saucepan; cook for 3 minutes, stirring, until the sauce has become slightly thickened (this is still a relatively thin sauce). Add the reserved pan juices and mix well. Just before serving, season with salt and pepper to taste and add the minced parsley and tarragon. Serve hot.