Adjust oven rack to lowest position, place 12-inch cast-iron skillet on rack, and heat oven to 500 degrees. Meanwhile, combine oil, rosemary, and garlic in bowl; set aside.
With chicken breast side down, use kitchen shears to cut through bones on either side of backbone; discard backbone. Flip chicken over, tuck wingtips behind back, and press firmly on breastbone to flatten. Pat chicken dry with paper towels, then rub with olive oil-rosemary-garlic mixture and season with salt and pepper.
When oven reaches 500 degrees, place chicken breast side down in hot skillet. Reduce oven temperature to 450 degrees and roast chicken until well browned, about 30 minutes.
Remove chicken from oven to gently flip chicken breast side up. Continue to roast chicken until breast registers 160 degrees and thighs register 175 degrees, about 10 minutes. Transfer chicken to carving board, tent loosely with aluminum foil, and let rest for 15 minutes. Carve chicken and serve.
Recipe by the minimalist table at https://www.daraboxer.com/theminimalisttable/cast-iron-roast-chicken-rosemary-garlic/