Step 1: In a medium bowl, whisk together the soy sauce, fresh lime juice, olive oil, minced garlic, grated ginger, brown sugar, red pepper flakes (if using), and black pepper. Stir until the brown sugar is fully dissolved.
Step 2: Place the flank steak in a shallow dish or a large, sealable plastic bag. Pour the marinade over the steak, ensuring it's completely coated. Marinate in the refrigerator for at least 2 hours, or preferably 4-8 hours (up to 12 hours).
Step 3: About 30 minutes before cooking, remove the marinated flank steak from the refrigerator to allow it to come closer to room temperature. Pat the steak dry with paper towels.
Step 4: Preheat your grill to high heat (about 450-500°F / 230-260°C) or a heavy-bottomed skillet (like cast iron) over medium-high heat until smoking gently. Add 1 tablespoon of high smoke point oil to the pan if pan-searing.
Step 5: Place the steak on the hot grill or in the hot skillet. Cook for 4-7 minutes per side for medium-rare to medium doneness (130-135°F / 54-57°C internal temperature).
Step 6: Transfer the cooked flank steak to a cutting board. Tent loosely with aluminum foil and let it rest for 5-10 minutes. This is crucial for tenderness and juiciness.
Step 7: Locate the grain of the meat and, using a sharp knife, slice the flank steak thinly against the grain. This ensures maximum tenderness.
Step 8: Arrange the sliced steak on a platter, garnish with fresh chopped cilantro or parsley, and serve immediately. Enjoy your perfectly cooked marinated flank steak!