In a medium bowl whisk together soy sauce, Worcestershire sauce, olive oil, brown sugar, minced garlic, black pepper and kosher salt until sugar dissolves and mixture is uniform.

Marinate the beef

Place the skirt (or flank) steak in a shallow dish or resealable bag. Pour the marinade over the beef, turning to coat both sides. Cover or seal and refrigerate for 30 minutes to 2 hours (30–45 minutes will give flavor; up to 2 hours for deeper flavor). Remove from fridge 20 minutes before cooking to come toward room temperature.

Preheat and season

Heat a heavy skillet or cast-iron pan over high heat until very hot (about 5 minutes). Pat the steak dry with paper towels; discard excess marinade. Lightly rub the steak with a little additional olive oil if the pan is very dry.

Sear the steak

Place the steak in the hot pan. Sear without moving for 3–4 minutes until a deep brown crust forms. Flip and sear the other side 3 minutes for medium-rare (internal temperature ~130–135°F / 54–57°C). For medium cook an additional 1–2 minutes. Adjust time for thickness and desired doneness.

Rest the steak

Transfer the steak to a cutting board and tent loosely with foil. Rest for 8–10 minutes to allow juices to redistribute.

Aromatics & Finish

Pan finish

While the steak rests, reduce heat under the skillet to medium. Add the butter and sliced shallot to the hot pan. Cook, stirring, 1–2 minutes until shallot softens and butter melts, scraping up any browned bits (fond) from the pan. Add lemon juice or red wine vinegar and swirl to deglaze, then remove from heat.

Slice and serve

Slice the rested steak thinly against the grain into 1/4-inch slices. Spoon the shallot-pan sauce over the sliced beef and sprinkle with chopped parsley. Taste and adjust with a pinch of salt or extra lemon juice if needed. Serve immediately.