In a large pan, heat cooking oil over medium heat. Cook onions for 5 minutes until softened.
Add garlic paste, ginger paste, and red chilli flakes. Saute for another 5 minutes.
Stir in curry powder, hoisin sauce, and oyster sauce. Season with salt and sugar. Bring to a boil.
Toss in the fish balls. Saute for a few minutes. Then stir in ½ cup water and let simmer for 10-15 minutes until reduced and the oil begins to separate.
Add the remaining water bit by bit accordingly if it gets too dry and to reach your desired consistency.
Garnish with scallions if using and serve immediately.
Notes
It's entirely up to you to first defrost the frozen fish balls or not. The recipe works fine if you cook the fish balls directly from frozen.
I used fried fish balls (yellow in colour), but steamed ones (white) are just fine as well.
Adjust the amount of water accordingly to your desired consistency. This recipe makes a pretty thick sauce. Add more water if you prefer it to be more liquid.