In a large skillet over meat heat, boil 1 ½ cup water then add the fish fillet. Let it cook for 10-15 minutes or until the fish is no longer pink. Remove from the pan, drain excess liquid and allow to cool.
Using a food processor, pulse the fish fillet until smooth. Add the onion, garlic, spring onion, salt, sugar and cornstarch. Gradually add the water and process again until well combined. The mixture should be homogenous but not too watery. Remove from the food processor and transfer it in a bowl.
In a pan over medium heat, add the cooking oil and wait until the oil is hot enough. Scoop 1-2 tbsp of the fish mixture, shape into a ball. Gently drop the fishball one by one and deep-fry for 3-4 minutes until lightly golden brown. Do not overcrowd the fishball in the pan to ensure that they are cook evenly. Remove the fish balls into the pan and arrange 4-5 pieces in each skewer.
To make the dipping sauce, heat a small saucepan over medium heat. pour the water, soy sauce and vinegar. Bring to boil then add the onion, garlic, sugar and cornstrach (diluted in ¼ cup water). Let it simmer until the sauce thickens.
Enjoy your homemade fish ball with sweet soy dipping sauce.