Thaw the turkey breast in the refrigerator (about 24 hours per 4 to 5 pounds). Heat the oven to 325°F and pat the turkey very dry.
Mash the butter with the garlic, rosemary, sage, thyme, parsley, lemon zest, salt, and pepper.
Loosen the skin and spread about two-thirds of the herb butter on the meat under the skin; rub the rest over the skin.
Scatter the onion and sliced zested lemon in the roasting pan, add the broth, and place the turkey on the rack, skin side up.
Roast at 325°F until the thickest part of the breast reads 165°F, about 2¼ to 3 hours for a 6 to 7 pound breast. Begin checking after about 1¾ hours, and tent with foil if the skin browns too fast.
Rest the turkey, loosely tented, for 20 minutes. Carve and serve with pan gravy.