Slice the onion and bell pepper into thin strips.
Cut the bulgogi beef into bite-sized pieces.
Heat cooking oil in a pan and sauté the onion and bell pepper.
Add the beef and season with soy sauce, sugar, minced garlic, and other seasonings; stir-fry thoroughly.
Once the bulgogi is cooked, remove from the heat and let it cool.
Place the bulgogi and vegetables on one slice of bread.
Generously add mozzarella cheese on top.
Cover with another slice of bread and toast lightly in a pan or sandwich maker.