Marinate the beef: In a bowl, mix the beef with cornstarch, sugar, soy sauce, salt, and pepper. Marinate for 15 minutes.
Cook the pasta: Bring a pot of water to boil, add in salt, and cook the spaghetti to al dente (2 minutes minus the package instructions recommend). Drain and reserve 1 cup of pasta water.
Prepare aromatics: While the spaghetti is cooking, prepare aromatics by mincing the garlic, and chopping the onion into dice. Set aside.
Sear the beef: Heat some oil in a pan over medium heat. Transfer the marinated beef to the pan, and sear for about 1 minute until golden-brown outside, while half-cooked inside. Remove from the heat, and set aside.
Sauté aromatics: Using the remaining oil from the beef in the pan, add in the aromatics and sauté until fragrant.
Stir fry with sauce: Transfer the cooked spaghetti to the pan, then add in soy sauce, oyster sauce, and dark soy sauce. Toss to coat, and season with salt & pepper to taste. Transfer the beef back to the pan, and stir fry together to mix well.
Finish with cheese: Turn down the heat to medium. Top with a sprinkle of shredded mozzarella, cover with a lid, and cook for another 1-2 minutes until melted.
Serve: Transfer to a plate, garnish with chopped coriander, and enjoy!