Remove the steak from the refrigerator and allow to come to room temperature 1 hour before cooking,
Heat a large cast iron skillet over medium-high heat, add the butter and allow to melt. Once melted, stir continuously, and watch as the water content cooks off, (the butter will sputter!), and the color goes from pale to golden tan, about 3 minutes.
Using the bone as a handle, place the steak in the pan away from you so you don't get splashed. Lower the heat to medium, add the rosemary to the pan, and allow the steak to cook undisturbed, 3-4 minutes per side, or until the internal temperature reaches 120° F. when measured with a digital meat thermometer. While the steak is cooking use a spoon to baste the steak with the rosemary-infused browned butter on both sides.
Remove the steak from the pan and transfer to a cutting board to rest for 15 minutes.
Use a sharp knife to slice along the bone. Then slice the remainder of the steak into ½-inch-thick slices and serve with Oven Roasted Shallots.