Pat the chicken dry with paper towels and season generously with kosher salt inside and out. Let the chicken sit for 1 hour, or in the refrigerator overnight.
Preheat the oven to 425°F and set a 12-inch cast iron skillet inside on the middle rack. Tie the legs of the chicken together with string or twine and pat dry with paper towels again. Drizzle with olive oil and lightly rub or brush the skin to cover it evenly. Season with a bit more salt and freshly ground black pepper.
In a medium bowl, toss the vegetables with half of the thyme leaves and a drizzle of olive oil and set aside.
Once the oven comes to temperature, drizzle 1-2 tablespoons of oil in the skillet. Very carefully place the chicken in the hot pan and quickly move out of the way in case the oil pops.
Carefully add the potatoes, onion, and thyme around the chicken and bake for 50-60 minutes or until a meat thermometer inserted into the thickest part of the thigh without touching the bone reads 155 to 160°F and the juices run clear.
Transfer from the oven and loosely tent with foil for about 15 minutes before serving.
