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1
In a 4-quart saucepan, heat the oil over medium heat.
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2
Add the onion and cook gently, stirring occasionally, for about 4 minutes, or until fragrant and soft.
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3
Add the tomatoes and salt, cover, and cook for 4 to 6 minutes, or until the tomatoes have collapsed into a thick mixture.
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4
Stir occasionally and, if necessary, lower the heat to prevent the tomatoes from sticking or scorching.
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5
Uncover and add the fish sauce and pork.
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6
Wield chopsticks or use a spoon to move the pork around the pan so that it breaks up into small pieces.
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7
This will make it possible to distribute the pork evenly among the bowls when serving.
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8
Add the water, raise the heat to high, and bring to a boil, using a ladle to skim and discard any scum that rises to the surface.
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9
Lower the heat to a simmer and cook, uncovered, for 15 to 20 minutes, or until the flavors have developed and concentrated sufficiently to produce a rich broth.
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10
If you are not serving the soup right away, turn off the heat and cover.
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11
Just before serving, return the soup to a simmer.
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12
Taste and add extra salt or fish sauce, if necessary.
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13
Turn off the heat.
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14
Pour the beaten egg onto the soup in a wide circle, and then stir gently to break it up into chiffonlike pieces.
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15
Ladle the soup into a serving bowl.
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16
Garnish with the cilantro and a generous sprinkle of pepper and serve immediately.
