Season beef with salt and pepper. Set aside.
Mix together soy sauce, sugar, oyster sauce, dark soy sauce, and water. Set aside.
Turn on wok or skillet on high. When pan is very hot, add oil. When oil is shimmery, add sliced sweet onion and allow to saute for 3-5 minutes. Then remove from pan.
Wipe down the pan. Turn on heat to high. When pan is hot, add in oil and wait until oil is shimmery. Then place in sliced steak and allow to sear for 30 seconds-1 minute on each side until beef is just cooked through.
Add cooked onions, green onion, butter, and sauce, and sauté for 3 minutes on high heat.
Add beef back into pan with satueed onion. Add in green onion. Add butter. Stir-fry and add soy sauce, sugar, and black pepper to taste.
Taste and adjust for seasoning.