Choose sirloin steak (about 1–1.5 inches thick)
Pat steak dry with paper towels
Season generously with salt and black pepper (optional: garlic powder or steak seasoning)
Preheat oven to 400°F (205°C)
Preheat a cast-iron or heavy skillet over high heat until very hot
Add 1–2 teaspoons high-smoke-point oil to the skillet
Sear steak 2–4 minutes per side without moving it
Sear edges briefly with tongs (optional)
Transfer skillet to the oven
Cook to desired doneness using a meat thermometer:
Rare: 125°F (52°C)
Medium-rare: 130–135°F (54–57°C)
Medium: 140–145°F (60–63°C)
Medium-well: 150°F (66°C)
Well-done: 160°F (71°C) and up
Remove steak from oven when target temperature is reached (it will rise 3–5°F)
Rest steak 5–10 minutes on a plate
Slice against the grain
Optional finish: add a small pat of butter during the last minute of searing; spoon over steak before resting