Pat the flank steak dry with paper towels. Trim any excess silver skin or large fat deposits, leaving a thin layer for flavor. If the steak is unevenly thick, use a meat mallet to gently pound the thicker parts to an even thickness (about 1/2–3/4 inch) so it cooks consistently.

Prepare marinade

In a medium bowl, whisk together olive oil, soy sauce, Worcestershire sauce, minced garlic, lime juice, ground cumin, smoked paprika, black pepper, kosher salt, and red pepper flakes (if using) until combined.

Marinate the steak

Place the steak in a large resealable plastic bag or shallow dish. Pour the marinade over the steak, pressing out excess air if using a bag, and massage the marinade into the meat so it's evenly coated. Refrigerate for at least 1 hour and up to 6 hours. (Do not exceed 8 hours for flank steak to avoid texture changes from the acid in the lime juice.) Remove from the refrigerator 30 minutes before grilling to bring to near room temperature.

Grilling & Finishing

Preheat grill

Prepare a charcoal or gas grill for high direct heat (about 450–550°F / 230–290°C). Clean and oil the grates well to prevent sticking.

Grill the steak

Remove the steak from the marinade and let excess drip off; discard remaining marinade. Place the steak on the hottest part of the grill. Grill 3–5 minutes per side for medium-rare (internal temperature about 125–130°F / 52–54°C). For medium, grill 5–7 minutes per side (135°F / 57°C). Avoid overcooking: flank steak is best medium-rare to medium.

Rest and finish

Transfer the grilled steak to a cutting board and tent loosely with foil. If using, place the butter on top of the warm steak to melt while it rests. Let rest 8–10 minutes to allow juices to redistribute.

Slice and serve

Slice the steak thinly against the grain into 1/4-inch slices. Arrange on a platter, sprinkle with chopped parsley, and serve with lime wedges for squeezing over the meat.