Heat the oil in a large pot over medium heat. Add the garlic and onion, and sauté until they are fragrant.
Add the sliced pork belly and stir-fry until it is browned and cooked through.
Add the shrimp and stir-fry until they are pink and cooked through.
Add the fermented fish sauce, shrimp paste, sugar, and water to the pot. Bring the mixture to a boil, reduce the heat to low, and let it simmer for 30 minutes.
Cook the rice noodles according to the instructions on the package.
To serve, place a portion of noodles in a bowl, and ladle the soup over it. Top with green onions, bean sprouts, mint leaves, and cilantro as desired.