In a large pot, bring water to a boil and add the beef shank, cooking for 5 minutes to remove impurities.

Drain the beef and rinse under cold water to clean.

In the same pot, heat vegetable oil over medium heat and add onions, garlic, and ginger, sautéing until fragrant.

Add the cleaned beef shank to the pot and stir to combine with the aromatics.

Pour in soy sauce, rice wine, rock sugar, star anise, and cinnamon stick, stirring well.

Add tomatoes and cover the mixture with water, bringing it to a boil.

Reduce heat to a simmer, cover, and let cook for about 2 hours until the beef is tender.

In a medium pot, bring water to a boil and cook the wheat noodles according to package instructions, then drain.

Blanch baby bok choy in the same pot of boiling water for 2 minutes, then remove and set aside.

Remove the beef from the soup and strain the broth to remove solids.

Return the beef and strained broth to the pot and adjust seasoning if necessary.

To serve, place noodles in bowls, ladle over the beef and broth, and top with bok choy and green onions.

Add chili paste for extra heat, if desired.