In a large pot, combine all the ingredients listed ”for the soup” and allow to reach a boil before lowering to a simmer. Remove any impurities from the soup and allow to simmer for 2 hours. Continue to add boiling water to the pot if the liquid reduces.
While waiting for the soup to cook, in a small bowl, add 2 tablespoon Mam Ruoc/fermented shrimp paste with 1.5 cups of warm water. We will let this sit so that the water and shrimp paste separate.
To make the saute/chili sauce: In a small pan, add ½ cup of cooking oil and 2 tablespoon of annatto seeds over medium high heat. After 2-3 minutes, the color from the annatto seeds will release into the oil. Using a mesh strainer, discard the seeds but keep the red oil. Once the seeds are removed, add the minced, shallot, minced garlic, and minced lemongrass to saute for 1 minute. Add the 4 tablespoon of Bun Bo Hue seasonings, fish sauce, and sugar. Gently stir to combine until it becomes a paste-like consistency. Taste and adjust as needed. Turn off the heat.
After 2 hours of cooking, we want to take out the beef shank and allow to cool in an ice water bucket to cool down. Using a mesh strainer, remove the pork bones, lemongrass, and white onion - discard this. Add half of the saute/chili sauce to the broth - this will help add the famous Bun Bo Hue color. Gently add the Mam Ruoc mixture - just the top clearer, separated part of the liquid (not the bottom). Add the remainder of the seasonings: 2 tablespoon fish sauce and 2 tablespoon chicken powder. Taste and adjust to your liking (more sugar, more fish sauce, more chicken powder).
Pat dry the cooked beef shank and slice into thin pieces. Same for the vietnamese pork ham (cha lua).
Cook the noodles according to package instructions.
Start assembling your bowls: add vermicelli noodles, sliced beef shank, sliced vietnamese pork ham (cha lua), boiled pork knuckles, sliced white/purple onion, sliced vietnamese corriander (rau ram). Ladle on the broth. Serve with side of fresh herbs/vegetables, lime, and thai pepper + fish sauce dip.
