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

Season and stuff

Generously season the cavity with 1 tsp kosher salt. Stuff the cavity with the halved lemon and quartered onion. In a small bowl combine softened butter, minced garlic, chopped thyme, 1 tbs olive oil, 1 tbs kosher salt and 1 tsp black pepper. Rub the butter mixture under the skin over the breasts and all over the outside of the chicken.

Truss and roast

Tie the legs together with kitchen twine and tuck the wing tips under. Place the chicken breast-side up on a rack set in a roasting pan or a rimmed baking sheet. Roast at 425°F (220°C) for 15 minutes to get 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 reads 165°F (74°C), about 45–60 more minutes depending on size. Baste once halfway through with pan juices.

Rest

Transfer the chicken to a cutting board and tent loosely with foil. Let rest 15–20 minutes before carving so juices redistribute.

Roasted Vegetables

Prep vegetables

While the chicken comes to room temperature, place carrots, halved potatoes and red onion wedges in a large bowl. Toss with 2 tbs olive oil, 1 tsp kosher salt, 1/2 tsp black pepper and chopped rosemary or thyme.

Roast

Spread vegetables in a single layer on a rimmed baking sheet. Roast in the same oven as the chicken: if the chicken goes in first at 425°F, roast vegetables for 20–25 minutes until golden and tender (they may finish while the chicken rests). If roasting alongside the chicken at 375°F, plan for 35–40 minutes, turning once halfway through for even browning.

Finish

Check seasoning and add a pinch more salt or a drizzle of olive oil if needed. Keep warm until the chicken rests and is carved.

Pan Sauce (optional)

Deglaze and reduce

Remove excess fat from the roasting pan, leaving browned bits. Place pan on stovetop over medium heat (or transfer bits to a skillet). Add 1/2 cup white wine or chicken stock, scraping up browned bits with a wooden spoon. Reduce by half, about 3–5 minutes.

Thicken and finish

Whisk in 1 tsp flour (or a teaspoon of cornstarch slurry) and simmer another minute until slightly thickened. Remove from heat and whisk in 1 tbs cold unsalted butter and chopped parsley for shine and flavor. Taste and adjust salt and pepper.

Serve

Carve the rested chicken, arrange on a platter with roasted vegetables and drizzle pan sauce over the carved meat or serve on the side.