Rinse the bangus belly fillets and pat dry with paper towels. Sprinkle with salt and pepper. Cover and set aside.
In a small sauce pan, stir together the soy sauce, kalamansi juice, garlic and sugar. Heat until boiling. Turn off the heat. Cover the pan and set the sauce aside.
Heat about two tablespoons of cooking oil in a frying pan. Lightly fry the onion rings just until softened. Scoop out and set aside.
Pour more cooking oil into the frying pan and heat to smoking point. You need enough oil to deep fry the bangus belly fillets so the depth of the oil should be at least two inches.
Dredge the seasoned bangus belly fillets in starch; shake off the excess. Fry in the hot oil (keep the heat on high) until nicely browned and the starch has formed a light and crisp coating. You may need to flip the fillets over depending in how well submerged they are during frying.
Assemble the dish. Arrange the fried fish in a platter. Scatter the onion rings on top. Pour the sauce over the fish and onion rings. You may optionally garnish the dish with torn mint leaves before serving. Bangus a la pobre is best paired with rice.