Preheat your oven to 425°F. Wash the baby potatoes and cut them into halves or quarters for even roasting.
Toss the potatoes with olive oil, salt, and pepper. Spread them out on a baking sheet in a single layer.
Roast the potatoes in the preheated oven for 20-25 minutes, until they are crispy on the outside and tender on the inside.
While the potatoes are roasting, pat the steak dry and season both sides with salt and pepper.
Heat a skillet over medium-high heat. Once hot, add the steak and sear for 3-4 minutes on each side for medium-rare, adjusting the time based on thickness and desired doneness.
In the final minute of cooking the steak, add the butter, garlic cloves (smashed lightly), and fresh rosemary to the pan. Tilt the pan and spoon the melted garlic butter over the steak to infuse it with flavor.
Remove the steak from the pan and let it rest for a couple of minutes. Slice against the grain if desired.
Plate the sliced steak with a side of crispy roasted potatoes, drizzle any remaining garlic butter over the top, and serve immediately.