In a medium bowl, whisk together the soy sauce, olive oil, sesame oil, honey, and rice vinegar until the honey is fully dissolved and the mixture is smooth. Add the minced garlic, grated ginger, and red pepper flakes, stirring well to distribute the aromatics evenly throughout the marinade. This creates a balanced flavor profile with savory, sweet, and spicy notes that will penetrate the meat.
Cut the flank steak into 1-inch cubes to maximize surface area for marinade absorption. Place the meat in a large resealable bag or shallow container and pour the marinade from Step 1 over it, ensuring all pieces are fully submerged. I recommend using a bag so you can shake it occasionally to redistribute the marinade—this helps the flavors penetrate more evenly than a static container. Refrigerate for at least 2 hours, though 6-12 hours is ideal for deeper flavor development.
Remove the steak from the refrigerator about 15 minutes before grilling to bring it closer to room temperature—this promotes more even cooking. Preheat your grill to medium-high heat (around 400°F). While the grill heats, remove the steak from the marinade, letting excess liquid drip back into the bag. Pick off any large garlic pieces clinging to the meat to prevent burning during grilling, though small bits are fine and will caramelize for extra flavor.
Place the steak cubes on the hot grill in a single layer, working in batches if needed to avoid overcrowding. Cook for 4-5 minutes per side without moving them too much—this allows a flavorful brown crust to develop. I like to listen for the sizzle to stop before flipping, which signals the meat has developed a good sear. Transfer the cooked meat to a clean plate and let it rest for 5 minutes to allow the juices to redistribute throughout the meat, keeping it tender.
Cut the rested steak against the grain into smaller, bite-sized pieces for maximum tenderness. Transfer to a serving platter and garnish generously with the sliced green onions, which add a fresh, bright contrast to the savory-sweet grilled meat. Serve immediately while the steak is still warm.