Bring a pot of water to a rolling boil and add the fresh udon noodles. Cook for 2-3 minutes until tender, then drain and set aside.

In a large skillet, heat oil over medium-high heat. Add sliced onion, minced garlic, and optional ginger. Stir-fry for 1-2 minutes.

Introduce the thinly sliced beef and stir-fry for another 1-2 minutes until brown.

Pour in the dark soy sauce, oyster sauce, tomato paste, and black pepper. Stir for 30 seconds.

Add the noodles and 1 cup of water, tossing for 2-3 minutes until noodles are coated.

Mix cornstarch with water and add to the pan, stirring until sauce thickens.

Fold in the chye sim, cooking for another minute until wilted.

Serve the noodles on plates, adjusting seasoning with black pepper, if desired.