Season the chicken thighs with salt and pepper and add them to a cold cast iron skillet the skillet, skin side down.
Turn the heat to medium and cook them like this, without moving them, until the fat has rendered out and the skin is deep golden brown and crisp, 15 to 30 minutes.
Reduce the heat if the skin begins to burn before it gets evenly golden brown.
Turn the thighs over and stir in the lemon slices, and rosemary sprigs into the pan juices, tucking the rosemary under the chicken.
Continue cooking the thighs until the meat closest to the bone is cooked through, about 15 minutes more.
Remove the rosemary and serve the thighs and lemony pan drippings with the lemon wedges.