In a large mixing bowl, add salt, pepper, and cornstarch. Toss the fish in the mixture and let them marinate for 15 minutes while you prepare other things
Combine all the ingredients for the seasonings in a bowl
Preheat a large skillet or wok. Add cooking oil. Add the garlic, ginger, and red chilis (if using) and stir fry for about 1 minute. Add the black bean paste and saute for another minute. Add water, seasonings, and the white part of the green onions. Bring to a simmer
Add the fish fillet and let them cook until they turn a white color. Stir them gently but don't stir too much that you break the fillet apart. The sauce will thicken too because there is cornstarch in the fish fillet marinade
Have a taste and adjust the seasonings of the sauce to your preference. Turn off the heat, garnish with the green parts of the green onion and transfer to a serving platter and serve immediately with rice
You can use any white fish fillet such as tilapia, catfish, snakehead fish, cod fish, etc.