Preheat oven to 350 degrees.
Heat 1/2 stick butter in cast iron skillet on medium high heat.
Season tops of thighs with a generous pinch of salt and pepper.
Once butter is melted and skillet is hot, add thighs top-side down.
Allow to cook for 3-4 minutes or until meat is brown and crispy.
Flip each thigh over and turn off heat.
Nestle rosemary sprigs in between thighs and top each thigh with a slice of lemon.
Place skillet in oven and cook for 15-18 minutes or until chicken is cooked through.