Pat dry turkey breast with paper towels. Season with salt and pepper, slipping salt under the skin where possible.
Combine butter, minced garlic, sage, rosemary, thyme, paprika, and parsley. Let it rest in the fridge until ready to use. You can make the herb butter a day or two in advance if desired.
Loosen the turkey skin by carefully running your fingers between the skin and the meat; the skin is very fragile, so be careful not to tear it.
Generously rub the turkey breast with the herb butter and season under the skin. Let it rest in the fridge for up to 24 hours.
Remove the turkey from the fridge an hour before cooking to bring it to room temperature.
Position the rack in the lowest part of the oven, remove any other racks.
Preheat your oven to 400ºF (200ºC).
Place onion, celery, and garlic in a 3-quart baking dish, then place the turkey breast on top, skin-side up.
Roast the turkey for 30 minutes, then reduce the heat to 325°F (160℃). Continue to roast until an instant-read thermometer registers 165°F (75℃) in the thickest part of the breast, about 1 hour or more, depending on the size of your turkey breast.
If the turkey is browning too fast, tent it with foil to prevent over-browning.
Remove the turkey from the oven and let it sit for 15-20 minutes before carving so the juices settle back into the meat. Reserve turkey drippings to make gravy. Discard the herbs and onions, and serve.