Preheat the oven to 325 degrees Fahrenheit. Move the oven rack one level below the center.
In a small bowl, mix together the butter, garlic, herbs and lemon zest and season the mixture with some salt and ground black pepper.
Rinse and pat dry the turkey breast with paper towels inside and out.
Using the back of a regular spoon or with your fingers, gently loosen the skin from the meat. Be careful not to tear the skin so the flavored butter doesn’t sip out when roasting.
Season the Turkey: Season the inside of the cavity with salt and ground black pepper. Smear about half of the herb butter underneath the skin, then smooth the skin back down and spread the remaining butter evenly over the skin. Season the turkey breast all over with salt and ground black pepper to taste.
Place the onions, carrots, celery, fennel (if using) and broth in a roasting pan large enough to accommodate the turkey breast comfortably. Place a rack over the veggies. If you don’t have a rack, simply place the turkey directly on top of the veggies.
Place the turkey breast, skin side up on the roasting rack. Roast for 2 to 2 1/2 hours or until an instant-read thermometer registers 165 degrees F when inserted into the thickest and meatiest parts of the breast. If the skin is over-browning, cover the breast loosely with aluminum foil.
Rest: When done, remove from the oven and allow to rest for about 10 - 15 minutes loosely covered with aluminum foil. Slice and serve with pan juices.