Pat the sirloin steak completely dry with paper towels to ensure a proper sear, then season both sides evenly with sea salt and black pepper.
Heat a heavy cast-iron skillet over medium-high heat and add the olive oil, swirling to coat the surface.
Add the asparagus spears to the skillet and sauté for 4-5 minutes until they are bright green and tender-crisp, then remove them from the pan and set aside.
Place the steak in the same hot skillet and sear undisturbed for 3-4 minutes to develop a golden-brown crust.
Flip the steak and immediately reduce the heat to medium, adding the ghee, minced garlic, and chopped rosemary to the pan.
Tilt the skillet slightly so the melting ghee pools with the garlic and rosemary, then use a large spoon to continuously baste the steak for 1-2 minutes.
Remove the steak from the skillet when it reaches your desired doneness and let it rest on a cutting board for 5 minutes before slicing against the grain.
Serve the sliced steak alongside the sautéed asparagus, drizzling any remaining garlic butter from the pan over the top.