Cut the ends of the fresh lemongrass, then tenderize it by laying it flat and rolling a rolling pin over it. Similar to rolling out dough. This helps releases the flavors inside the lemongrass.
Place the remaining chopped shallots, ginger, white onion, garlic and tenderized lemongrass in a pot with 2 tbsp of vegetable oil and sauté for 3 minutes.
Add the marinated beef short ribs to the pot with the aromatics to sear the meat.
After the meat is seared, add 2 L of water so that all the meat is fully submerged.
Bring the pot to a boil. As the broth boils, you will see a thin film (a.k.a. bot in Vietnamese). Take a spoon and remove the thin film. This will make the broth look clearer and tastes better.
Turn the heat to medium low, close the lid and let the beef short ribs simmer for 2 hours.
After the beef is cooked, add the chicken broth and Bún Bò Huế paste. Mix well then turn the heat to low to continue simmering the beef ribs.
Add fish sauce...
Add rock sugar and mix well. Let the pot simmer for another 10 minutes, then remove the lemongrass stalks before serving.