In a medium mixing bowl, combine soy sauce, sesame oil, brown sugar, minced garlic, grated ginger, chopped green onion, black pepper, and sesame seeds. Mix well, then add thinly sliced beef and marinate for at least 20 minutes. Sauté the beef for 3-5 minutes until caramelized and browned, then cool and chop.

Take 2 tablespoons of the cooked short-grain rice and flatten it in your palm. Place a portion of chopped bulgogi and a small amount of shredded mozzarella in the center. Shape the rice around the filling to form a ball. Repeat with remaining rice, beef, and cheese.

Set up three bowls: one with all-purpose flour, another with beaten eggs, and the last with panko breadcrumbs mixed with a pinch of salt and garlic powder. Roll each arancini in flour, dip in beaten eggs, then coat with panko. Let rest for about 10 minutes.

Heat vegetable oil in a deep frying pan to 350°F (175°C). Fry arancini in small batches for 3-4 minutes until golden brown. Remove with a slotted spoon and drain on paper towels.

Arrange fried arancini on a platter. Serve with dipping sauces like gochujang aioli or soy glaze, and consider adding pickled vegetables for crunch.