Step 1: In a medium bowl or a large resealable plastic bag, whisk together the soy sauce, olive oil, lime juice, minced garlic, brown sugar, red pepper flakes (if using), and chopped cilantro or parsley. Ensure the sugar is dissolved.
Step 2: Pat the flank steak dry thoroughly with paper towels. This helps the marinade adhere better and promotes a better sear. Place the flank steak into the bowl or bag with the marinade, ensuring it’s completely coated.
Step 3: Marinate the flank steak in the refrigerator for at least 2 hours, but preferably 4-6 hours for maximum flavor and tenderness. You can even marinate it overnight (up to 12 hours). Do not marinate for more than 24 hours.
Step 4: Remove the flank steak from the refrigerator about 30-45 minutes before cooking to allow it to come closer to room temperature. This promotes more even cooking. Discard any remaining marinade.
Step 5: Preheat your grill (or a heavy cast-iron skillet) to high heat. You want it screaming hot to get a beautiful sear. Lightly oil the grill grates or skillet to prevent sticking.
Step 6: Place the flank steak on the hot grill or skillet. Cook for 4-6 minutes per side for medium-rare (internal temperature of 130-135°F / 54-57°C) or 6-8 minutes per side for medium (internal temperature of 135-140°F / 57-60°C). Adjust cooking time based on the thickness of your steak and desired doneness. A meat thermometer is highly recommended.
Step 7: Once cooked to your desired doneness, transfer the flank steak to a cutting board. Tent it loosely with foil and let it rest for at least 5-10 minutes. This allows the juices to redistribute throughout the meat, ensuring a tender and juicy result.
Step 8: After resting, slice the flank steak against the grain into thin strips. Identify the direction of the muscle fibers and slice perpendicularly to them. This shortens the fibers, making the steak incredibly tender and easy to chew. Serve immediately.