This boneless turkey breast cooks fast and is exceptionally tasty, thanks to the fennel stuffing.
Melt butter in a large skillet over medium heat. Add onion and garlic, and cook, stirring occasionally, until onion is soft and translucent, about 4 minutes. Add fennel; cook, stirring, until tender, about 4 minutes. Transfer mixture to a large bowl. Stir in the bread, thyme, rosemary, parsley, and 1 1/4 cups stock. Season with salt and pepper. Set stuffing aside.
Preheat oven to 375 degrees. Place the turkey, skin side down, on a clean work surface. Using a sharp knife, remove the tenderloins. To butterfly the turkey, slice vertically through the right side of breast, starting at thickest part and slicing almost to edge without cutting through (it should resemble a book, with a flap in the middle). Spread open, keeping both sides attached, and gently press down to flatten. Repeat on left side of breast. Cover with plastic wrap. Using a meat mallet or heavy skillet, pound meat until thickness is uniform.
Season turkey with salt and pepper, then spread stuffing lengthwise down middle. Fold both sides of turkey over stuffing. Using kitchen twine, tie turkey at 1-inch intervals to completely encase stuffing and form a long cylinder.
Transfer the turkey to a roasting pan. Pour 1 cup stock into pan. Brush turkey with oil, and season with salt and pepper. Roast, basting with pan juices every 30 minutes, until well browned and an instant-read thermometer inserted into thickest part of the turkey registers 165 degrees, about 1 3/4 hours. If the skin begins to get too dark, loosely tent pan with foil (add remaining 1 cup stock if pan gets too dry). Transfer the turkey to a carving board, and let rest 20 minutes before slicing. Garnish with rosemary sprigs and orange slices, if desired. Serve hot or at room temperature.
This recipe can be doubled or tripled to serve larger groups; if you expect guests to arrive throughout the day, roast the turkey breasts at thirty-to forty-five-minute intervals.