Slice the beef thinly and marinate it with soy sauce, sugar, minced garlic, and sesame oil to make bulgogi marinade.
Let the beef marinate in the sauce for 20 minutes.
Cut the onion, bell pepper, and mushrooms into thin strips.
Heat cooking oil in a pan and stir-fry the marinated beef with the vegetables.
Place the stir-fried mixture and mozzarella cheese on top of a tortilla.
Cover with another tortilla and cook on both sides in a pan until golden.
Cut into bite-sized pieces and serve.