Combine soy sauce, sugar and juice from 1 lemon. Mix well.
Marinate the skirt steak in the soy sauce mixture for 30 minutes.
Segregate the beef with the marinade.
Heat the cooking oil and then sear the marinated beef for 3 minutes.
Pour-in the remaining marinade, water, and sesame oil. Stir and then let boil. Simmer for 30 minutes Add water as necessary.
Let the liquid reduce to half. Pour-in cornstarch diluted in water. Stir until the sauce thickens.
Add salt and pepper.
Add the onion. Cover and cook for 2 minutes.
Transfer to a serving plate. Sprinkle sesame seeds on top.
Serve. Share and enjoy!