Rinse beef shank/bones/tendon in cold water. Drain.
Fill up a large pot of cold water. Bring to a boil.
Add the beef shank/bones/tendon to the boiling water. Let it boil vigorously for 10 minutes.
Drain the beef bones and rinse with cold water to remove any scum/impurities.
Transfer the beef shank/bones/tendon to an Instant Pot.
Prepare the spices:
In a dry frying pan over low heat, add the star anise, cinnamon stick, black cardamom pods, fennel seed and Sichuan peppercorn. Toast until fragrant.
Transfer the toasted spices into a cheesecloth and tie it up with kitchen string.
Place the spice pack into the Instant Pot.
Add the seasonings:
To the Instant Pot, add dou ban jiang (red fermented broad bean paste) [if using], black bean paste, dark soy sauce, light soy sauce, shaoxing wine, 5-spice powder, garlic, ginger, green onions, tomatoes, rock sugar, and sea salt.
Add the beef stock and cover with additional water so that all the beef/tendon are submerged.
Cook the beef:
For Instant Pot: Place the lid on, ensure the sealing valve is set, and set to MEAT setting, HIGH PRESSURE for 65 minutes. Naturally release pressure.
For crockpot: simmer on low for 6-8 hours, until beef shank and tendon are soft and tender.
Remove the beef shank (remove any bones) and beef tendon. Place the meat aside.
Strain out the soup and discard the spice packet and vegetables (tomatoes, garlic, ginger, etc).
Taste the soup for seasoning and adjust accordingly.
Skim the fat off the soup. (You can also chill the soup in the fridge to do this easily).
Cook the veggies:
In the meantime, cook the bok choy in a large pot of boiling water for about 1-2 minutes. Don't overcook the bok choy, since it will continue to cook slightly with the heat of the soup. Remove the bok choy and set it aside.
Cook the noodles:
Next, using the same boiling water, cook about 1.5 lb of dry or fresh wheat noodles according to the directions on the package.
Drain the noodles and give them a quick rinse with cold water.
Assemble the bowls:
Portion the noodles, beef shank/beef tendon, and bok choy into bowls.
Ladle the hot beef soup over top and garnish with green onions, cilantro, pickled mustard greens and extra chili oil (if desired).