Start by putting the pork belly in a large pot covered with water. Boil over high heat until it is fork tender, 1 hour 30 minutes.
Next, place the pork in a strainer and drain to remove all of the water, 30 minutes.
In a deep fryer, add the oil and preheat to medium-high heat. Fry the pork until golden brown, 30 to 45 minutes. (The thicker the slab of belly, the longer it will take to fry.) Remove and drain.
Chop the pork belly into 1-centimeter-square pieces. In a medium mixing bowl, add the pork belly, onions, tomatoes, fish sauce and salt, and toss all together. Plate and add green onion for garnish before serving.
Professional Recipe: This recipe was provided by a chef, restaurant or culinary professional and makes a large quantity. The Food Network Kitchens chefs have not tested this recipe in the proportions indicated and therefore cannot make any representation as to the results.