Grab a large pot and heat it up. Add water and bring it to a boil. Boil the beef shank briefly, then remove and rinse it. Set the beef aside.
Toss the chopped sweet onion, garlic, and white part of the leek into the same pot. Let it soften a bit. Add the ginger, dried chilies, and star anise. Give it a minute, then stir in the spicy bean paste and tomato paste. The smell will let you know it’s working.
Pour in the soy sauces and vinegar. Return the beef to the pot. Toss in the sweet onion whole, the stock cube, and just enough water to cover everything. Add salt and sugar lightly—you can adjust later. Cover partially and simmer for about 1.5 hours while bringing to a gentle boil.
Boil noodles separately, then drain and rinse if you like a springy bite.
Scoop noodles into bowls. Ladle over hot soup and beef. Add scallions and chili oil. Sit down, breathe, enjoy.