Place the aromatics in a food processor and finely chop them (or you can manually chop them by hands)
Cut the fish fillet into large chunks, about 2-inch pieces. Don't cut too small or the fish will break apart easily when you cook them
Preheat a large wok or skillet. Add cooking oil. Add ginger and garlic mixture. Stir fry until fragrant, about 2 minutes. Pour in the chicken stock and seasonings and bring to a boil. Add the fish fillet pieces in and let them cook until the fillet changes into opaque white color and cooked through. It shouldn't take too long, about 2 minutes or less
Carefully dish out the fish fillet from the stock into a serving platter. Have a taste for the stock, add more salt if needed. Bring it to a boil and then give the cornstarch mixture a stir and pour in. Continue to stir until the stock is thickened. Gently add the fish fillet pieces back. Gently stir to let the fillet pieces coated in the sauce. Turn off the heat. Sprinkle with chopped green onion. Serve immediately