Preheat your air fryer to 400°F to ensure a high-heat sear for the steak and potatoes.
Scrub the russet potato clean and slice it into 1/2-inch thick wedges for optimal crunch.
In a small bowl, toss the potato wedges with 0.5 tsp olive oil, 0.25 tsp garlic powder, and half of the sea salt and black pepper.
Pat the sirloin steak dry with a paper towel and rub both sides with the remaining oil, garlic powder, rosemary, salt, and pepper.
Arrange the potato wedges in the air fryer basket in a single layer and cook for 10 minutes, shaking the basket halfway through.
Place the seasoned steak into the basket alongside the potatoes and air fry for 8 to 10 minutes, flipping the steak once for a juicy medium-rare finish.
Remove the steak and let it rest on a cutting board for 5 minutes before slicing against the grain to serve with the crispy potatoes.