To create your Korean Steak Sandwich, follow these straightforward steps:

1. Prepare the Marinade: In a bowl, whisk together soy sauce, brown sugar, sesame oil, garlic, ginger, rice vinegar, gochujang, and cornstarch until well combined.
2. Marinate the Steak: Place the flank steak in a resealable bag or shallow dish and pour the marinade over it. Seal or cover, then refrigerate for 30 minutes.
3. Cook the Steak: Preheat a grill or stovetop skillet on medium-high heat. Remove the steak from the marinade and cook for 5-7 minutes per side for medium-rare, or until your desired doneness is reached.
4. Rest the Steak: Remove the steak from the heat and let it rest for 5 minutes. This allows the juices to redistribute for a tender bite.
5. Slice the Steak: Thinly slice the steak against the grain for maximum tenderness.
6. Prepare the Bread: While the steak rests, toast the sandwich bread lightly in a toaster or on a skillet for a crisp outer layer.
7. Assemble the Sandwich: On the toasted bread, layer the sliced steak, shredded carrots, cucumber, and chopped lettuce. Add a dollop of mayonnaise if desired.
8. Garnish and Serve: Sprinkle with green onions for added flavor. Cut the sandwich in half for easy handling, and serve immediately.

These steps will guide you to create an incredible Korean Steak Sandwich that is sure to impress everyone at the table!