Dice the herbs and them to a medium-sized mixing bowl. Press the garlic cloves into the bowl. Season generously with salt and pepper.
Add the softened butter into the herb mixture.
Place about 70% of the herb butter into a plastic storage bag. Press it towards the bottom of the bag, forming a log shape. Place the bag in the refrigerator to harden.
Dice the carrots and celery into large chunks. Place the veggies into the bottom of a large roasting dish with a detachable roasting rack. Pour the broth directly over the veggies. Place the wire roasting rack inside the dish.
Rinse the turkey in the sink, remove the giblets, and pat it dry with paper towels. Place the turkey inside the roasting dish on top of the wire roasting rack. Carefully, use your fingers to loosen the skin away from the flesh, being extra careful not to rip it.
Slice the herb butter into 1/4-inch thick coins. Lift the skin and evenly distribute the butter coins underneath it. Season the outer turkey skin very generously with salt and pepper. Tie the turkey legs together with twine.
Remove the skin from the two onions and cut them in half. Stuff the onion pieces inside the cavity of the turkey. Spread the leftover butter on top of the turkey. Bake the turkey uncovered at 350°F for 3-5 hours depending on the weight. Note: A 12-14 pound turkey will take about 3 to 3 and a half hours to bake. Tack on an extra 13 minutes per pound if your turkey is heavier.
Allow the turkey to rest at room temperature for 20 minutes before carving it. Save all the drippings and broth at the bottom of the roasting pan for gravy. Slice and enjoy!