Sauté aromatics. Heat oil in a large soup pot over medium. Add garlic, onions, and half the tomatoes. Cook until onions turn translucent and fragrant.
Brown the beef. Drop in the neck bones, turning until the exterior loses its raw look. This quick sear layers in extra flavor.
Build the broth. Pour in rice water. Bring to a gentle boil, skim off foam, then lower the heat. Cover and simmer 45–60 minutes, or until beef starts to soften. Add plain water along the way to keep everything submerged.
Season & add root veg. Stir in the tamarind mix and beef powder. Taste—add more mix if you want it punchier. Slip in taro and daikon; simmer 10–15 minutes until a fork slides through easily.
Load remaining veggies. Add eggplant, the rest of the tomatoes, and green peppers. Cook 5 minutes. Drop in okra and string beans; simmer another 5 minutes.
Finish & serve. Splash in fish sauce, adjusting saltiness. Stir in water spinach; once it wilts (about a minute), kill the heat. Ladle over hot rice, making sure everyone gets a bit of everything.