Pat the sirloin dry with paper towels. Trim any excess fat and, if the steak is thicker than 1 inch, slice it horizontally to an even thickness of about 3/4–1 inch so it cooks evenly.

Make the marinade

In a bowl combine the minced garlic, soy sauce, olive oil, lemon juice, Worcestershire sauce, brown sugar, black pepper, and kosher salt. Whisk until sugar dissolves and mixture is homogenous.

Marinate

Place the steak in a shallow dish or resealable plastic bag. Pour the marinade over the steak, turning to coat all surfaces. Marinate in the refrigerator for at least 30 minutes and up to 4 hours. If short on time, marinate at room temperature for 20–30 minutes.

Bring to room temperature

Remove the steak from the refrigerator 20–30 minutes before cooking so it comes close to room temperature. Pat off excess marinade (leave a thin glaze) and lightly season again with a pinch of salt and pepper if desired.

Cooking & Finishing

Preheat pan

Heat a large heavy skillet or cast-iron pan over medium-high heat until very hot, about 3–5 minutes. Add 1 teaspoon of oil (from the olive oil allotted) to just coat the pan and swirl.

Sear the steak

Place the steak in the hot pan and press down gently to ensure full contact. For a 3/4–1 inch sirloin, sear 3–4 minutes on the first side without moving it to develop a deep brown crust. Flip and sear the second side 2.5–3.5 minutes for medium-rare (internal temp 130–135°F / 54–57°C). Adjust time for thickness and desired doneness.

Baste with butter and garlic

In the last 1 minute of cooking, reduce heat to medium-low, add the 2 tablespoons unsalted butter to the pan and spoon the melted butter over the steak continuously (basting) for about 30–60 seconds to enrich flavor. If there is any residual minced garlic from the marinade, use it while basting; otherwise add a tiny pinch of fresh minced garlic to the butter for a final garlic hit.

Rest the steak

Transfer the steak to a cutting board and tent loosely with foil. Let it rest for 5–10 minutes so juices redistribute. Resting time depends on steak thickness; thicker cuts benefit from 8–10 minutes.

Slice and garnish

Slice the steak thinly against the grain for maximum tenderness. Sprinkle chopped parsley and, if desired, red pepper flakes for a little heat. Spoon any pan juices or butter over the slices before serving.