Boil the Udon Noodles: Fill a large pot with water and bring it to a rolling boil. Add fresh udon noodles and blanch for about 2-3 minutes until just tender. Drain and rinse under cold water.

Sauté Aromatics: In a large frying pan, heat 2 tablespoons of oil over medium-high heat. Add sliced onions, minced garlic, and ginger. Sauté for 3-4 minutes until onions soften.

Cook the Beef: Add shabu shabu beef to the pan and stir-fry for about 2-3 minutes until just browned. Ensure even cooking.

Add the Sauce Ingredients: Pour in dark soy sauce, oyster sauce, tomato paste, and black pepper into the pan. Stir and add 1/4 cup of water. Let bubble for 2 minutes.

Combine Noodles and Sauce: Gently add reserved udon noodles and toss to coat in the sauce. Stir for 1-2 minutes.

Add Vegetables: Stir in chye sim and cook for an additional 1-2 minutes until wilted.

Thicken the Sauce: If desired, mix cornstarch with water for a slurry. Add gradually while stirring and cook for an additional minute.

Serve Hot: Plate the noodles immediately and serve hot, garnished with additional black pepper if desired.