Add beef cubes to a pot filled with water, Bring it to a full boil. Remove any impurities that appears on the surface. Drain then put the beef back into the pot.
Prepare the soup
Heat up oil in a wok (or a deep frying pan) over medium heat. Stir in garlic, Sichuan chilli bean paste and chilli powder.
Cook until fragrant (do not burn). Pour in 1500ml (6½ cups) hot water. Cover and leave to simmer for 5 minutes.
Simmer the beef
Through a sieve, pour the spiced soup into the beef pot. Add ginger, scallions and all the other spices (see note 3).
Bring the soup to a boil then simmer for 1 to 1.5 hours. Once the beef becomes tender cooked, discard the ginger, scallions and spices.
Cook the noodles
Cook the noodles in another pot with plenty of water. At the end of the process, quickly blanch spinach (or pak choi) in the same pot. Drain and rinse the noodles and vegetable under cold water for a few seconds.
Assemble the dish
Place the noodles and vegetable in serving bowls. Pour in the broth then place the beef on top. Sprinkle coriander to garnish. Serve immediately (see note 4).
Notes
1. Different brands of Sichuan chilli bean paste (aka spicy Doubanjiang, 辣豆瓣酱) may vary in saltiness, hotness and texture. Adjust the volume accordingly. Best to use Pixian Douban, a well-known variety, which needs to be coarsely chopped prior to cooking.2. You may use fresh noodles or dried noodles. In terms of volume, I usually cook about 80-100g / 2.8-3.5 oz dried noodles for each person. Follow the instructions on the package. Please note that you are not supposed to boil the noodles in the beef broth since the starch coming off the noodles will change the flavour and the consistency of the broth.3. If available, use a closed tea strainer to keep spices in while simmering. Later you will be able to discard them easily.4. You may freeze the meat and soup in the freezer for up to 1 month. Defrost in the fridge then heat up again prior to serving.