Preheat the oven to 425°F (180°C).
Thoroughly pat the chicken's skin dry all with a paper towel and drizzle olive oil on both sides. Generously rub half of the herb seasoning mixture onto the chicken, both skin-side and cavity side.
¼ cup olive oil, 1 whole chicken, spatchcocked
Toss the halved baby potatoes, onion slices, and garlic with the rest of the seasoning, lemon juice, and oil in the cast iron pan.
2 lb baby potatoes, Juice of 2 lemons, 1 medium yellow onion, 2 heads garlic
Roast the chicken for 50-60 minutes, turning halfway through for even cooking, until the skin is deeply golden brown (and if using a meat thermometer, the internal temperature reads 160℉ to 165℉ in the thickest part of the thigh without touching the bone). Once done, remove from the oven.
Take the chicken from the cast iron pan and place it on a clean cutting board. Remove the potatoes from the pan as well with a slotted spoon to a serving plate. You'll be left with a pan sauce in the pan! Let chicken rest for 10 minutes to redistribute juices.
Carve the chicken on the cutting board and serve with potatoes and the pan sauce.