Preheat your oven to 425°F.
Clean the baby potatoes, then cut them in halves if they are larger than bite size. Toss them in a small amount of olive oil (optional) and sprinkle with salt, pepper, and chopped rosemary.
Spread the potatoes on a baking sheet and roast in the preheated oven for about 20-25 minutes or until they are fork-tender and slightly crispy.
While the potatoes are roasting, pat the sirloin steak dry with a paper towel and season both sides generously with salt and pepper.
Heat a skillet over medium-high heat. Once hot, sear the steak for about 3-4 minutes on each side for a medium-rare finish (adjust time to your preferred doneness).
In the last minute of cooking, add the butter, garlic (crushed or minced), and a sprig of fresh rosemary to the pan. Spoon the melted herb-garlic butter over the steak continuously.
Remove the steak from the skillet and let it rest for a few minutes.
Plate the rested steak alongside the roasted baby potatoes and drizzle any remaining herb-butter from the pan over the top.