Place the beef cubes in a pot of water and bring it to a rolling boil to remove impurities. Drain and transfer the beef back to the pot.
Heat cooking oil in a wok or deep pan over medium heat, then add minced garlic, Sichuan chilli bean paste, and chilli powder. Stir and cook until the aroma develops without burning.
Pour in the hot water, cover the wok or pan, and let it simmer gently for 5 minutes to infuse the flavors.
Strain the spiced soup through a sieve into the pot with the beef. Add ginger, scallions, bay leaf, star-anise, cinnamon piece, Sichuan pepper, fennel seeds, and sugar.
Bring the soup to a boil, then reduce heat and simmer uncovered for 1 to 1.5 hours until the beef is tender. Remove and discard the ginger, scallions, and whole spices.
Cook the noodles in a separate pot with plenty of boiling water. At the end of cooking, quickly blanch the spinach or pak choi in the same pot. Drain and rinse both noodles and vegetables under cold water briefly.
Divide noodles and vegetables into serving bowls. Ladle the hot broth over them, top with tender beef cubes, and garnish with fresh coriander. Serve right away for the best taste.
