Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Cut the russet potato into even wedges and place them in a bowl.
Toss the potatoes with half of the olive oil, the chopped rosemary, garlic powder, and a pinch of salt and pepper.
Spread the potatoes in a single layer on the baking sheet and roast for 25-30 minutes, flipping halfway through, until golden and crispy.
While the potatoes roast, pat the sirloin steak dry with paper towels and season both sides with the remaining salt and pepper.
Heat the remaining olive oil in a cast-iron skillet over medium-high heat until shimmering.
Place the steak in the pan and sear for 3-4 minutes per side for medium-rare, or until it reaches your desired doneness.
During the last 2 minutes of cooking, add the smashed garlic cloves and the whole rosemary sprig to the pan, tilting the skillet to spoon the aromatic oil over the steak.
Remove the steak from the pan and let it rest on a cutting board for at least 5 minutes to lock in the juices.
Slice the steak against the grain and serve immediately alongside the hot rosemary garlic wedges.