Preheat your oven to 425°F and line a baking sheet with parchment paper.
Toss the baby potatoes with olive oil, salt, and pepper. Spread them evenly on the baking sheet.
Roast the potatoes in the oven for 20-25 minutes until tender and golden, shaking halfway through.
While the potatoes roast, let the sirloin steak come to room temperature and pat dry with a paper towel.
Season the steak generously with salt and black pepper on both sides.
Heat a heavy skillet or cast-iron pan over medium-high heat. Once hot, sear the steak for about 3-4 minutes per side for medium-rare, adjusting time as needed.
In the last minute of cooking, add the unsalted butter, minced garlic, and rosemary sprig to the pan. Spoon the melted herb-garlic butter over the steak.
Remove the steak from the pan and allow it to rest for a few minutes before slicing.
Plate the sliced steak alongside the roasted baby potatoes, garnish with fresh parsley, and enjoy.