In a large bowl, combine soy sauce, sesame oil, brown sugar, gochujang, minced garlic, minced ginger, and black pepper. Mix well until the sugar is dissolved.

Add the thinly sliced beef sirloin to the marinade. Toss to coat the beef evenly. Cover the bowl with plastic wrap and let it marinate in the refrigerator for at least 30 minutes, or up to 2 hours for more flavor.

Heat vegetable oil in a large skillet over medium-high heat. Once hot, add the marinated beef slices in a single layer (you may need to do this in batches). Cook the beef for about 2-3 minutes on each side, or until browned and cooked through.

Remove the beef bites from the skillet and sprinkle with sesame seeds and chopped green onions.

Serve hot with the gochujang dipping sauce on the side.

In a small bowl, whisk together the gochujang, soy sauce, rice vinegar, honey, sesame oil, minced garlic, and minced ginger until smooth. Set aside as a dipping sauce.