Thinly slice the bell peppers and dice the red onions.
Clean and slice the beef steak into thin strips.
Marinate the beef strips for 15 minutes with salt, oil, and cornstarch.
In a bowl, prepare the sauce by combining light soy sauce, black pepper powder, cornstarch, water, and dark soy sauce.
Cook the udon noodles in water for 2 minutes or according to the package instructions. After cooking, let it cool in cold water and set aside. While cooking, carefully separate the noodles to stop them from sticking together.
Heat a pan with oil for stir-frying. Stir-fry the beef strips until the color changes to light brown. Set aside.
In the same pot with less oil, stir-fry the diced onions and bell peppers over medium heat for 2 minutes.
Add the udon noodles. Give the sauce another mix and pour it into the pan.
Stir-fry for an additional 2 minutes and add the beef strips.
Continue stir-frying until the sauce is evenly mixed and starts to thicken.