1. In a large pot, add beef shank, pork hock, lemongrass, ginger, onion, and water. Bring to a boil.
2. Reduce heat and simmer for about 1.5 hours, until meat is tender.
3. Remove meat from the pot, strain the broth, and return it to the pot.
4. Add fish sauce and salt to the broth. Adjust seasoning to taste.
5. Cook rice noodles according to package instructions. Drain and set aside.
6. To serve, place noodles in bowls, top with beef and pork, and ladle hot broth over. Garnish with bean sprouts, herbs, lime wedges, and chili oil if using.
Notes
Feel free to adjust the spice level by adding more or less chili oil.