Preheat your oven to 425°F.
Thoroughly wash the baby potatoes and cut them in halves if they are larger. Place them in a bowl and drizzle with olive oil, salt, and black pepper. Toss to coat well.
Spread the potatoes on a baking sheet in a single layer and roast for 20-25 minutes until they turn golden and crispy on the edges, stirring halfway through.
While the potatoes are roasting, take the sirloin steak out of the refrigerator and let it come to room temperature. Pat dry with paper towels.
Season the steak liberally with salt and black pepper on both sides.
Heat a heavy skillet or cast iron pan over medium-high heat. Once hot, add the steak and sear for about 3-4 minutes on each side for medium-rare, or adjust timing to desired doneness.
In the last minute of cooking, reduce the heat to medium, add the unsalted butter and minced garlic (or crushed garlic cloves), and spoon the melting garlic butter over the steak to infuse rich flavor.
Remove the steak from the pan and let it rest for 5 minutes to allow the juices to redistribute.
Plate the steak alongside the crispy roasted potatoes, drizzle any remaining garlic butter from the pan over the steak, and serve immediately.