Preheat the oven to 450°F. Prepare your vegetables: Chop the carrots and celery into thirds. Quarter the onion. Halve the garlic cloves.
Prep the chicken: Remove from the package and take out the giblets packet inside. (Freeze giblets for stock/gravy or toss.) Then pat the chicken dry with a paper towel, making sure to get under the wings and legs.
Take one garlic clove half and rub the inside of it all over the chicken. Drizzle olive oil over the top and rub on all sides - make sure to get under the wings and legs again. Salt and pepper the outside.
Place the chicken, breast side up, in the cast iron pan or roasting pan. Stuff the veggies and garlic cloves inside the cavity and close it up. If you have stainless steel meat pins, use these to loosely close up the cavity and to pin the legs down. If you don't have them, just try to bring the legs close to the body and close up the cavity.
Put the chicken in the hot oven. Cook for 15 minutes, then lower the temperature to 350°F and cook for another 15 minutes per pound. (So a 5 lb chicken would cook for 15 minutes at 450F, then at 350F for an hour and 15 minutes)
The chicken is done when it has reached 165°F with a meat thermometer in the thickest part of the thigh. Careful not to hit the bone when measuring the temperature.
Remove the chicken from the oven and cover with foil. Let rest for 10 minutes, then carve and enjoy!
Notes
*Trussing the Chicken: If you have stainless steel meat pins, use these to loosely close up the cavity and to pin the legs down. If you don't have any, no worries! Just try to bring the legs close to the body and close up the cavity. The chicken may roast a few minutes faster.* Tucking the wingtips: Tucking the wingtips up under the legs helps to keep them from burning, but if they pop out, not to worry! If you notice them browning too much during cooking, cover them lightly with aluminum foil to prevent further browning.