In a large pot, combine the beef and water. Bring to a boil over medium-high heat, then reduce to a simmer. Skim off any foam that rises to the surface.
Add the quartered onion and tomatoes to the pot. Simmer for about 1.5 to 2 hours, or until the beef is tender.
Once the beef is tender, add the radish, green beans, and eggplant. Cook for an additional 5-7 minutes until the vegetables are tender.
Stir in the tamarind paste or sinigang mix and fish sauce. Adjust the seasoning with salt and pepper to taste.
Add the green chili peppers and simmer for another 2-3 minutes.
Remove from heat and let it sit for a few minutes before serving. Garnish with fresh cilantro or green onions.