In a mixing bowl, combine soy sauce, brown sugar, sesame oil, garlic, ginger, green onion, and black pepper. Add the sliced beef and mix well. Let it marinate for at least 15 minutes.

Heat a skillet over medium-high heat. Add the marinated beef and stir-fry for about 5-7 minutes until cooked through.

In a bowl, place a serving of cooked rice. Top with stir-fried beef, carrots, cucumber, and spinach. Sprinkle with sesame seeds before serving.