Marinate the Beef:

• In a large bowl, combine light soy sauce, mirin, fish sauce, dark brown sugar, minced garlic, chopped fresh ginger, ground black pepper, and sesame oil.

• Add the thinly sliced flat iron steak to the marinade, ensuring all pieces are well-coated.

• Cover and refrigerate for at least 30 minutes to allow the flavors to meld.

Prepare the Vegetables:

• While the beef is marinating, thinly slice the onion, julienne the carrot, and chop the green onions.

Preheat the Oven:

• Preheat your oven to 475°F (245°C).

• Line a large sheet pan with parchment paper or lightly grease it to prevent sticking.

Assemble on the Sheet Pan:

• Spread the marinated beef evenly across the prepared sheet pan.

• Distribute the sliced onions and julienned carrots over the beef.

Cook:

• Place the sheet pan in the preheated oven and cook for about 10-12 minutes, or until the beef is cooked through and slightly caramelized.

• For added char and flavor, you can switch to broil for the last 1-2 minutes, keeping a close eye to prevent burning.

Garnish and Serve:

• Once cooked, remove from the oven and sprinkle the chopped green onions over the beef.

• Serve hot over steamed rice or alongside lettuce leaves for wrapping, if desired.