Start by cutting 2 pounds of pork belly into 2½-inch cubes. Peel and quarter one medium onion. Set these aside.
Place the pork in a large pot and add 8 cups of cold water. Turn the heat to medium and bring to a boil. When bubbling, use a spoon to carefully remove any gray foam that rises to the top of the water.
Once the water looks clear, add the quartered onion, ½ teaspoon cracked black peppercorns, and 1 tablespoon fish sauce. Let it boil for 1-2 minutes. Lower the heat, cover the pot, and cook for about 45 minutes until the pork is tender when pierced with a fork.
While waiting, prepare your vegetables. Peel and remove seeds from one small squash (kalabasa), then cut it into 2-inch pieces. Wash two bunches of pechay and separate the leaves.
When the pork is tender, add the squash pieces to the pot. Cook for about 6 minutes or until you can easily pierce them with a fork but they still hold their shape. Add the pechay leaves and cook for just one minute until they turn bright green but are still crisp.
Taste the soup and add salt if needed. You can also add more fish sauce instead of salt if you prefer.
Serve hot in bowls, with steamed rice on the side. For extra flavor, prepare a small dish of fish sauce mixed with chopped chilies as a dipping sauce for the meat.
The soup should be enjoyed while hot, with clear broth and tender meat that easily pulls apart with your spoon.
Tip: To make sure your soup stays clear, avoid stirring it too much and always remove foam that rises to the top during the first boil.*