Combine softened butter, garlic, rosemary, thyme, and parsley. Refrigerate it until ready to use. You can make it a day or so ahead.
Rinse the turkey legs with water, then pat dry with paper towels. Season them with salt and pepper or turkey seasoning.
Remove the herbed butter from the fridge, and bring it to room temperature or microwave for about 20 seconds.
Loosen the skin by running your fingers carefully between the skin and the meat. The skin is fragile, so be careful not to tear it.
Generously rub the legs with the herb butter mixture and refrigerate until ready to cook (up to 24 hours).
Preheat oven to 400ºF (200ºC).
Place onion, celery, and rosemary and thyme stems in a 3-quart baking dish or tray, depending on the size of the turkey legs. Then, place the legs on top of the vegetables.
Roast the turkey legs for 30 minutes, then reduce the heat to 325°F (160℃). Continue roasting until an instant-read thermometer reads 165°F (75℃) in the thickest part of the turkey leg (30-40 minutes more, depending on their size).
If the turkey browns too fast, tent it with foil to prevent over-browning.
Remove the baking dish from the oven and let it rest for 10-15 minutes before carving so the juices settle back into the meat for juicier turkey.
Reserve turkey drippings for gravy (if desired) and enjoy with mashed potatoes or roasted vegetables.