Preheat your oven to 400°F (200°C) and line a small baking sheet with parchment paper.
Wash and cube the red potatoes into 1/2-inch pieces, then toss them in a bowl with half of the olive oil and a pinch of the sea salt and black pepper.
Spread the potatoes in a single layer on the baking sheet and roast for 20-25 minutes, tossing halfway through, until they are golden and crispy.
While the potatoes roast, pat the sirloin steak dry with a paper towel and season both sides with the remaining salt and pepper.
Heat the remaining olive oil in a heavy-bottomed or cast-iron skillet over medium-high heat until it begins to shimmer.
Place the steak in the hot skillet and sear for 4-5 minutes per side for medium-rare, or until it reaches your desired level of doneness.
In the final minute of cooking, add the minced garlic and chopped fresh rosemary to the pan, spooning the infused oil over the steak to baste it.
Remove the steak from the pan and let it rest on a cutting board for at least 5 minutes before slicing against the grain.
Serve the sliced steak alongside the roasted potatoes, drizzling any remaining pan juices over the meat.