Peel shrimps, reserving the shell. Do not discard. Pat dry with paper towels.
In a large pot, bring about one third pot of water to boil. Add pork ribs and pork belly. Continue to boil for 5 minutes. Remove pork ribs and pork belly with a pair of thongs. Discard water.
Fill the same pot with 10 cups (2.4 liters) of water. Bring it to a boil. Return pork ribs and pork belly to the pot. Lower heat and allow it to simmer.
Heat 4 tablespoons vegetable oil in a medium sized pan. Add sliced shallots and fry until fragrant and golden brown in color, about 5 minutes. Remove with strainer and set aside.
Add blended onion and fry for about 3 minutes. Add chili paste and continue to fry for another 2 minutes. Transfer to a small bowl.
Add remaining 1 tablespoon vegetable oil into the same pan. Add shrimp shells and fry until shells turn pink, about 3 minutes. Remove and allow fried shrimp shells to cool slightly. Blend the shells in a food processor until fine. Transfer to a filter bag.
Place filter bag and 2 tablespoons of cooked chili paste (keep the remaining as condiment) to the soup. Season with salt and allow soup to simmer for 1½ hours.
After half an hour of simmering, remove pork belly. When cool enough to handle, slice thinly and set aside.
In the meantime, fill a separate pot half full of water. Bring to a boil. Scald bean sprouts for about 20 seconds. Remove with a metal strainer. Do the same for water spinach. Discard water.
Refill pot with water and bring it to a boil. Add noodles and allow it to cook for about 3 minutes or according to packaging instructions. Do not overcook. Remove with metal strainer.
When soup is done, remove pork ribs and filter bag with processed shrimp shells. Discard filter bag. Remove meat from pork ribs or discard. Add fish sauce and shrimps to the soup and allow it to cook for about 3 minutes until shrimps curl and turn pink. Remove and set aside.
Place a portion of noodles, bean sprouts, and water spinach in a bowl. Pour soup over noodles and vegetables. Top with shrimps, sliced streaky pork, and reserved chili condiment. Serve immediately.
1. Additional topping may include slices of hard cooked egg. 2. For a richer soup, save as much fresh shrimp heads and shells as you can in the freezer until ready to be used.