First, I pat the chicken dry (both inside and out) to help skin crisp properly. Then I season the cavity with a bit of salt and pepper, and carefully stuff in half of the onion quarters, both halves of the garlic head, the lemon quarters, and celery pieces.
In a small bowl, whisk softened butter, Dijon mustard, lemon juice, oregano, and rosemary until smooth. I gently loosen the skin over the breasts and spread about a third of the butter mixture beneath it. The rest I slather all over the skin, making sure I get into every crease around wings and legs.
Preheat your oven to 425 °F (220 °C). Place the chicken breast-side up on a bed of the remaining onion quarters in your roasting pan (or on a rack). Begin roasting at 425 °F for 30 minutes, then lower to 350 °F (175 °C) and continue for around 1 hour, or until the internal temperature of breast and thigh registers 155–160 °F (it’ll rise a bit during resting).
Once done, remove the chicken and tent it loosely with foil. Let it rest about 15 minutes before carving—this helps juices redistribute and keeps the meat moist.