Peel the potatoes and slice off the ends. Cut the potatoes into 1-inch thick rounds. Season both sides of the potatoes generously with salt and pepper.
Heat a cast-iron skillet (or a heavy-duty oven-proof pan) over medium-high heat. Add the oil and half of the butter. Sear the potatoes on one side until golden brown, about 4 minutes. Flip the potatoes and add the rosemary and garlic.
Cut the remaining butter into cubes and scatter on top of the potatoes. Transfer the skillet to the oven and cook until fork-tender, about 25 minutes. Garnish with parsley and spoon the pan sauce over the potatoes to serve.