Heat 2 Tbsp Oil (or more) in a pot over medium-high heat. Sear the beef until some parts are lightly browned. Don't cook it all the way through. Do this in batches if your pot is small. If you crowd the pan, it will get watery and it's difficult to develop the browning.
Add the aromatics (onion, garlic, green onion, ginger, chili) and stir-fry until the onion turns slightly transparent and softens. Take the beef out first if the pot is too crowded.
Add 2-3 Tbsp Cooking Rice Wine or some splashes of water to deglaze the pot. Then mix in 3 Tbsp Douban Sauce, 6 Tbsp Soy Sauce, and 1/2 Tbsp Sugar.
When the sauce starts to thicken and smells amazing, stir in the 5 Tbsp Tomato Puree and let it cook down until it thickens slightly.
Pour in just enough water to cover all the beef and bring it to a boil. Be careful not to add too much—too much water will dilute the broth and make it harder for the beef to absorb flavor.
Add spice bag to the pot simmer the soup on low heat for 1.5 hour.
After an hour, remove the spice bag, ginger, chilli skin, and large chunks of spring onion. Add the radish and carrot chunks. Pour in just enough water to cover everything, then let it simmer for another hour or until the beef is super tender.
While the soup is simmering, boil your Noodles. Just prepare the portion you are going to serve and always prep fresh ones before servings because it would get soggy in the soup.
Taste the soup and adjust the saltiness by adding more water, soy sauce, or salt as needed. I usually would need at least a cup of water and a tsp of salt.
To serve, combine the soup with the noodles and optionally garnish with thinly sliced green onions.
Enjoy!