Keep using the residual oil left in the wok that's left after removing the beef; that will be plenty to stir-fry and cook the rest of the dish.
Keep the heat on low and add the ginger and the onion. You can raise the heat to medium if it doesn't seem hot enough. Stir-fry the ginger and onion until fragrant, or about 15-20 seconds.
Add the tomato wedges, then stir-fry for 20-30 seconds.
Add ketchup (2 tbsp) and mix it in. Then, add water (3 cup). This may seem like a lot of water, but remember, this amount of sauce has to go with that large plate of crispy noodles! Turn the heat to high, then season the sauce.
Add sugar (2 tbsp) and salt (1 tsp), then taste and adjust it to your liking if necessary.
Once you're happy with the flavor, add the beef back in and turn the heat to low. Season with oyster sauce (1 tbsp).
Then, mix up a slurry with water (4 tbsp) and cornstarch (2 tbsp). As always, pour it in slowly while constantly stirring, and stop when you're happy with how thick the sauce is. You may not need to use all the slurry. The sauce is done! Turn the heat off.
Ladle the tomato and beef sauce right over your dish of crispy pan-fried noodles. Serve and eat immediately for maximum crunch. Enjoy!