Begin by preparing all ingredients. Cut beef into 2-inch cubes, peel and clean taro roots, cut onions into wedges, bruise lemongrass stalks by pounding them lightly, and separate kangkong leaves from stalks. Cut watermelon into 1-inch cubes and set aside.
Fill a large pot with water and place beef inside. Bring to a boil and cook for 10 minutes to remove impurities. Drain the beef and rinse under running water until clean. This step ensures a clear, clean-tasting broth.
In a clean large pot, arrange watermelon cubes in a single layer at the bottom. Place the cleaned beef on top of the watermelon layer. Add onion wedges, ginger slices, and bruised lemongrass. If using fresh tamarind, wrap it in muslin cloth and tie ends securely. Add water just enough to cover everything.
Bring to a boil, then lower heat to maintain a gentle simmer. Cook for 1 hour, allowing the beef to become tender and the watermelon to infuse the broth with sweetness.
Add the peeled taro roots and continue simmering for 30 minutes until they're tender when pierced with a fork. The taro will help thicken the broth naturally.
Add the sinigang mix or, if using fresh tamarind, squeeze the muslin bag several times to extract all the juice. Remove and discard the lemongrass stalks. Taste and adjust sourness according to your preference.
Add green chilies and okra. Cook for 5 minutes until the okra is just tender but still bright green. Don't overcook the vegetables to maintain their texture.
Add kangkong leaves, cover the pot, and turn off the heat immediately. The residual heat will cook the greens perfectly while keeping them crisp.
Season with freshly ground black pepper and fish sauce to taste. Let the soup rest for 5 minutes before serving to allow flavors to settle.
Serve hot in individual bowls alongside steaming white rice. Prepare small dishes of fish sauce and sliced chilies as condiments, allowing everyone to adjust flavors to their liking.