Heat oil in a pan over medium heat. Add the sliced onions, minced garlic, and optional ginger. Stir-fry until fragrant, about 2-3 minutes. This step enhances the flavor base of your dish.
Add the shabu shabu beef slices to the pan and cook until browned, approximately 3-4 minutes. Make sure not to overcrowd the pan to achieve a nice sear.
Mix in the dark soy sauce, oyster sauce, tomato paste, and black pepper with the beef. Pour in the water to create a sauce. For convenience, you can pre-mix these ingredients in a bowl before adding them.
Add the udon noodles to the sauce, gently separating them until they are no longer in a block. Allow the sauce to come to a boil, which should take about 2-3 minutes.
Once boiling, add the leafy greens, such as chye sim, and stir gently until they are wilted, about 1-2 minutes.
Gradually add the cornstarch slurry to the pan, stirring continuously until the sauce thickens to your desired consistency, which should take about 1-2 minutes. Serve hot and enjoy!
Notes
Recipe notes: No alcoholic drinks allowed, please substitute with a non-alcoholic option.