Place the turkey breast or breast half in a baking dish or Dutch oven. (See notes about breast up or breast down.)
In a small dish, combine the thyme, tarragon, and sage. Lay the pats of butter on a plate and season them generously with the herbs.
Loosen the skin of the breast and place the butter pats under the skin. Sprinkle the skin generously with salt and pepper.
Place the herb sprigs and onion in the pan around the turkey breast.
Bake for 30 minutes at 425°. Reduce the heat to 325° and continue baking for 45 to 60 minutes, or until it reaches an internal temperature of 165°. Allow the turkey breast to rest before slicing.
Slice the turkey breast and serve.
Notes
Nutritional values are approximate and are based on ⅙ of a six-pound turkey breast.
Refrigerate leftovers promptly and use within 4 days.
The size of turkey breast you need will depend on how many people you are serving. You will want 1 to ¼ pounds of turkey breast per person if it's your sole protein or if you want leftovers.
If yours is a large whole turkey breast, you may want to roast it breast side down for the first hour and then turn it over to brown the skin and continue cooking until done. Do this carefully as the turkey breast can be quite heavy.
Use the drippings, bones, and aromatics left in the pan to make turkey stock.