Preheat the oven to 425°F (220°C) with a rack in the center; set a sturdy roasting pan nearby.
Pat the chicken dry thoroughly with paper towels. Season inside and out with the kosher salt and black pepper, then rub the skin all over with the unsalted butter until evenly coated.
Stuff the cavity with the lemon, garlic, and thyme sprigs; tuck the wing tips under and tie the legs loosely with kitchen twine.
Scatter the onion, carrot, and celery in the roasting pan to form a bed. Set a rack over the vegetables (or rest it directly on them), then place it breast-side up.
Roast at 425°F until the skin is deep golden and an instant-read thermometer in the thickest part of the thigh registers 160–165°F, 60–75 minutes, rotating the pan halfway through.
Transfer to a cutting board and rest, uncovered, 15 minutes so the juices redistribute.
Set the roasting pan over medium heat on the stovetop. Add the chicken stock and simmer, scraping up browned bits, until reduced slightly and glossy, 3–5 minutes; strain if desired.