Preheat your oven to 425°F. While the oven warms up, wash the baby potatoes and cut them in halves to ensure even roasting.
Toss the potatoes with olive oil, salt, and pepper, and spread them out on a baking sheet in a single layer.
Roast the potatoes in the preheated oven for 20-25 minutes, or until they are golden brown and crispy, stirring halfway through.
Season the sirloin steak generously with salt and pepper on both sides.
Heat a skillet over medium-high heat. Once hot, add the steak to the pan, and sear for about 3-4 minutes on each side for medium-rare, adjusting the time to your preferred doneness.
During the last minute of cooking the steak, add the butter and finely chopped garlic to the pan. Spoon the melted garlic butter over the steak to infuse it with rich, savory flavor.
Remove the steak from the skillet and let it rest for a few minutes to retain its juices.
Plate the rested steak alongside the crispy roasted potatoes, drizzling any remaining garlic butter from the pan over the top, and serve immediately.