For the soup stock
- 2 tablespoons fish sauce, plus more to taste
- 2 teaspoons kosher salt
- 2 teaspoons black pepper
- 1 1/2 pounds beef round, cut in half lengthwise
- 3 1/2–4 pounds beef bones
- 8 cups low-sodium chicken broth
- 6 cups water
- 2–3 fresh lemongrass stalks, cut into large pieces and lightly smashed
- 1 large yellow onion, peeled and quartered
- 1 tablespoon granulated sugar
- 8 ounces dried flat rice noodles (see Notes)
- Thinly sliced onions
- Chopped fresh cilantro
- Chopped fresh green onions
Other optional garnishes
- Hoisin sauce
- Asian fish sauce
- Hot sauce such as Sriracha sauce
- In a small bowl, mix together the fish sauce, salt, and pepper. Place the beef round pieces in a medium bowl and rub the beef all over with the marinade. Cover the bowl and let the beef marinate in the refrigerator for at least 2 hours.
- Place the beef bones and marinated beef round in a large stock pot. Pour in chicken broth and water. Bring the mixture to a boil over medium-high heat, skimming out the impurities and froth. Reduce heat to low. Add the lemongrass, onion, and sugar. Partially cover the pot and simmer until the beef round is tender, about 1 1/2 hours. Using tongs, remove the beef round from the stockpot and transfer it to a storage container and place in the refrigerator.
- Continue simmering the soup stock for 2-3 more hours, skimming out the impurities and froth as needed.
- Strain the broth through a fine-mesh strainer into a new pot. Let the soup settle for a few minutes and then skim as much fat as possible off the top (see Notes). Season with fish sauce and salt to taste.
- Cook the noodles in a large saucepan of boiling water until just tender, 6 to 8 minutes. Drain the noodles and rinse well until the water runs clear. Divide the noodles among serving bowls. Top the noodles with some thinly sliced onions.
- Remove the beef round from the refrigerator and cut it into thin slices. Place the slices on top of noodles and onions, dividing the beef equally among the serving bowls.
- Bring the strained broth to a boil over high heat.
- Ladle the boiling broth over the sliced beef in each bowl. Top with some sliced green onions and cilantro. Add other garnishes as desired.
Dried flat rice noodles are sometimes labeled as Banh Pho or Pad Thai noodles. They often come in different widths. For this soup, I recommend the small or medium width.
The most effective way to defat the stock is to refrigerate it, uncovered, until it is well chilled and the fat has solidified on top. Lift off and discard the fat.