Add the rosemary, 2 tablespoons olive oil, mustard, honey, salt, pepper and beer to a large bowl or baking dish. Add the chicken, refrigerate for at least 30 minutes and up to 12 hours.
Preheat oven to 375.
Heat the remaining 1 tablespoons olive oil in a cast iron skillet until hot but not smoking. Add the potatoes, cook until browned on cut sides, but not cooked through, about 5 minutes, remove from heat.
Place the chicken on top of the potatoes, pour ½ cup of the chicken marinate over the chicken.
Bake at 375 until chicken and potatoes are cooked through, about 25 minutes.