In a bowl, combine the salt, thyme, sage, paprika, pepper, and ground mustard until evenly incorporated.
Remove the turkey from its packaging. From the bird's cavity, remove the neck and the bag containing the gizzards and heart. (You can keep these items to make stock or discard them.)
Use a few paper towels to dry off the skin of the bird and inside the cavity.
Lift the skin over the turkey breast (on the side closest to the legs) and slide your hand under the skin separating it from the breast meat. Do this on both sides of the breast.
Insert three pats of butter under the skin on one side fo the turkey breast spreading them around evenly. Repeat on the other side.
Sprinkle the dry rub all over the turkey - breast, legs, wings, anything you can see. Using your hands press the rub into the skin a little. If you have any extra rub sprinkle it inside the cavity.
Place the onion halves and garlic cloves inside the turkey's cavity.
Transfer the bird to your roasting pan. (I like to use a pan with a rack to keep the bottom of the turkey from getting soggy.)
Take a good-sized piece of foil and place it over the turkey breast. (You want to make sure to the foil piece is big enough to fit over the whole breast.) Press down and mold the foil to the breast., let the ends stick out if the foil's a bit big.
Place the turkey in the oven and roast for 2 hours. Carefully remove the foil from the turkey. Increase the oven temperature to 400 degrees and cook the turkey for another hour, or until the internal temperature of the breast reaches 165 degrees F and the skin is golden and crispy.
Remove the turkey from the oven and let it rest briefly before carving.
If your bird is bigger then 12 pounds, add 15 minutes of cook time at 325 degrees F for each additional pound. (ex. a 14-pound turkey will roast for 2 1/2 hours at 325 degrees F and then 1 hour at 400 degrees F.)