Place the pork, water, onion, tomatoes, and fish sauce in a large soup pot or Dutch oven. Bring to a boil over high heat. Skim off any gray foam that rises to the surface for a clearer broth.
Reduce the heat to maintain a gentle simmer. Cover partially and cook for 40 to 45 minutes, or until the pork is becoming tender. Stir occasionally and add a splash of water if the liquid level drops too much.
Stir in the tamarind soup base mix until dissolved. Taste the broth carefully; different brands vary in saltiness and sourness.
Add the daikon and cook for 8 minutes. Add the green beans and eggplant, then simmer for 7 to 10 minutes more, until the vegetables are tender and the pork is very tender.
Turn off the heat and stir in the spinach just until wilted, about 1 minute. Taste and add more fish sauce if needed. For a more sour broth, add a little more tamarind mix according to the package directions.
Ladle the soup, pork, and vegetables into bowls. Serve hot with steamed white rice, if desired.