Preheat the oven to 425°F (220°C). Remove the chicken from the refrigerator 30 minutes before roasting to bring it closer to room temperature for even cooking. Pat the chicken thoroughly dry inside and out with paper towels.

Season cavity

Season the cavity generously with about 1/2 tablespoon kosher salt and a few grinds of black pepper. Insert the halved lemon, smashed garlic cloves, and rosemary and thyme sprigs into the cavity along with the quartered onion.

Butter under skin

Gently loosen the skin from the breast by sliding your fingers between skin and meat, creating pockets. Distribute the softened butter under the skin over both breasts and rub any remaining butter over the outside of the skin. Drizzle 1 tablespoon olive oil over the exterior and rub to coat.

Final seasoning

Sprinkle the remaining 1.5 tablespoons kosher salt and 1 teaspoon freshly ground black pepper evenly over the outside of the chicken. Tie the legs together with kitchen twine and tuck the wing tips under the body to prevent burning.

Roast

Place the chicken breast-side up on a rack set in a roasting pan or on top of the prepared vegetable bed (see next section). Roast in the preheated oven 20 minutes at 425°F (220°C) to develop color, then reduce oven temperature to 375°F (190°C) and continue roasting until an instant-read thermometer inserted into the thickest part of the thigh (not touching bone) reads 160–165°F (71–74°C). For a 3.5–4 lb chicken this will typically take another 45–55 minutes (total roast time ~65–80 minutes).

Rest

Remove the chicken from the oven and transfer to a cutting board. Tent loosely with foil and let rest 15–20 minutes; carryover heat will bring the internal temperature to 165°F (74°C) and allow juices to redistribute.

Carve

After resting, remove twine and carve the chicken: remove legs and thighs, then breasts and wings. Slice breasts against the grain and serve with pan juices or roasted vegetables.

Vegetable Roasting Bed (optional but recommended)

While the oven preheats, toss the carrots, celery and halved potatoes with 2 tablespoons olive oil, 1 teaspoon salt and 1/2 teaspoon black pepper in a bowl. Spread the vegetables in an even layer in the bottom of the roasting pan. Place the prepared chicken on a rack set over the vegetables or directly on top of them so vegetables roast and absorb chicken drippings.

When the chicken is done and resting, transfer the roasted vegetables to a serving platter. If vegetables need browning, place under the broiler for 2–3 minutes watching carefully.

Finishing (optional)

While the chicken rests, place the roasting pan with drippings over medium heat on the stovetop (or pour drippings into a small saucepan). Add 1/2 cup pan juices or chicken stock and scrape up browned bits with a wooden spoon. Simmer 2–3 minutes to reduce slightly. Taste and adjust seasoning. Strain if desired and stir in chopped parsley. Serve alongside carved chicken.