Preheat oven to 425°F.
In a large saucepan over medium heat, melt butter. Add onion, celery and fennel and sauté, stirring occasionally, until soft and translucent, about 7 minutes. Add mixed fresh herbs, parsley, and salt and pepper; stir to combine. Transfer to a large bowl. Add the stuffing cubes 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 tenderloin 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 loose cylinder. Using kitchen twine, tie the rolled turkey breast crosswise at 1-inch intervals. Rub the outside of the turkey breast with 1 tablespoon of olive oil and season with salt and pepper. Repeat with the other tenderloin. (You will have some stuffing left over.)
Place 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 and continue roasting until skin is crisp and an instant-read thermometer inserted into the thickest part of the breasts registers 165°F, about 40 minutes more.
Transfer 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 alongside turkey gravy.
Recipe adapted from: https://www.williams-sonoma.com/recipe/rolled-stuffed-turkey-breast.html.