Lightly season fish fillet pieces with salt and pepper, set aside. Heat cooking oil and sesame oil in a pan over medium heat.
Add garlic, onion, saute until fragrant then add black beans. Cook for about a minute then add a cup of water; season with fish sauce ( or soy sauce if you want), and a little sugar and black pepper; bring to a boil then add the fish fillet pieces.
Cook for about 5 minutes, then invert the fish fillet pieces and cook for about 3-5 minutes more.
Make the slurry by mixing cornstarch with 1 tablespoon water then add a little of this to the pan ( you may use all of it or not depending on how you want the consistency of your sauce); bring to a boil, check the seasoning then turn off the heat.
Sprinkle scallions and serve with rice. Enjoy! :)
You may use bangus (milkfish) belly or boneless bangus fillet, but I suggest that you fry it first, it just taste better that way.