First, clean your bangus and pat it dry with paper towels. Mix flour, salt, and pepper in a shallow dish to make your coating.
Season your fish pieces with salt and pepper, then coat each piece in the seasoned flour. Shake off any extra flour.
Heat oil in a large pan over medium-high heat until hot. The oil should be about one inch deep. Fry the fish for 3-4 minutes on each side until golden brown but not fully cooked. Place on paper towels to drain.
Heat one tablespoon of oil in a clean pan over medium heat. Cook the sliced onions until they turn clear, about 2 minutes. Add the garlic and ginger, cooking until you can smell their aroma.
Add the chopped tomatoes to the pan. Press them gently with your spoon as they cook to help release their juices. Cook for 3-4 minutes until the tomatoes soften.
Add the fish sauce and let it cook for a minute. Pour in one cup of water and bring to a gentle simmer. Add your rinsed black beans and stir gently. Let this cook for 3-5 minutes until the sauce thickens slightly.
Carefully place your fried fish in the pan. Spoon the sauce over the fish pieces. Lower the heat and let everything cook together for 3-5 minutes, or until the fish is fully cooked and covered with sauce. Try not to stir too much – instead, just spoon the sauce over the fish.
Taste the sauce and add salt and pepper if needed. If it's too thick, add a little water. Serve your Bangus sa Tausi hot with steamed rice, making sure to spoon plenty of sauce over both.
Remember: Don't skip rinsing the black beans before cooking, and handle the fish gently to keep it from breaking apart. Your sauce should be thin enough to spoon over rice but thick enough to coat the fish.