For the butter, mince the garlic. Mix all of the ingredients together in a small bowl.
Preheat the oven to 170C/325F.
Rinse the turkey inside and out. Pat the skin dry and place in a roasting rack inside a large roasting pan.
Chop the onion and celery. Slice the lemon into wedges. Mince the garlic.
Stuff the cavity of the bird with the veggies from step 4.
Loosen the skin of the bird by putting your hands between the skin and the breast meat working your way down the leg. Take some of your butter mixture and rub it thoroughly under the skin over the breast and leg meat. Coat as much as you can. It's easier if the butter is at room temperature.
Pour the olive oil all over the turkey and sprinkle the salt and pepper all over. Tie the legs of the turkey together with kitchen twine or oven proof string. Put a probe thermometer into the thickest part of the breast (if you have one).
Place the turkey breast side up into the oven on the middle rack and bake for 2.5 hours to 3 hours. Halfway through, baste the turkey juice all over the skin. At 2.5 hours, check the thermometer. It should register at 160F/71C. If you don't have a probe thermometer, open the oven and check with a handheld thermometer. If it's not there yet, cook it a bit longer (10-40min).
Once the breast meat registers at 160F/71C, take it out of the oven and cover the dish with aluminum foil for 30 minutes. This will finish cooking it.
After that, you can start carving it!