1. In a large pot, bring water to a boil. Add pork knuckles and cook for 5 minutes. Drain and rinse the pork knuckles.
2. Refill the pot with fresh water and bring to a boil. Add pork knuckles, onion, garlic, fish sauce, salt, sugar, and black pepper. Simmer for 1.5 hours until pork is tender.
3. In another pot, cook the thick rice noodles according to package instructions. Drain and set aside.
4. To serve, place noodles in bowls, ladle the soup with pork knuckles over the noodles. Garnish with green onions and cilantro.
Notes
For extra flavor, you can add a dash of lime juice and some chili slices.