Preheat an oven to 425°F
In a large saucepan over medium heat, melt the butter. Add the onion and celery and sauté, stirring occasionally, until soft and translucent, about 7 minutes. Add the mixed fresh herbs, parsley, and salt and
pepper to taste and stir to combine. Transfer to a large bowl. Add the stuffing and 3 cups of the stock and stir to combine; add more stock if needed to form a moist stuffing. Set aside.
Place 1 butterflied turkey breast on a work surface, with
a long side nearest you, and season with salt and pepper. Mound 3 cups of the stuffing in the center, leaving a 1-inch border on each long side. Fold the long side nearest you over the stuffing to enclose, gently pressing on the filling, then roll to form
a cylinder; do not roll it too tightly or the filling will slip out of the ends. Using kitchen twine, tie the rolled turkey breast crosswise at 1-inch (2.5-cm) intervals. Rub the outside of the turkey breast with 1 Tbs. of the olive oil and season with salt
and pepper. Repeat with the other turkey breast half. (You will have some stuffing left over.)
Place the remaining stuffing in the bottom of a roasting pan. Set the turkey breast halves, skin side up, on top. Roast for 25 minutes, then reduce
the oven temperature to 350°F. Continue roasting until the skin is crisp and an instant-read thermometer inserted into the thickest part of the breasts registers 165°F about 40 minutes more.
Transfer the turkey breasts to a carving board,
cover loosely with aluminum foil and let rest for 5 to 10 minutes. Remove the twine and cut the turkey into 1/2-inch slices. Serve immediately with the turkey gravy alongside.