If rice cakes are frozen or hard, soak them in warm water for 10–15 minutes; drain and set aside.
In a small bowl, whisk together gochujang, gochugaru, light soy sauce, caster sugar and minced garlic until smooth.
In a medium saucepan over medium heat, combine water or light vegetable stock with the white parts of the spring onions and bring to a gentle simmer.
Stir the sauce mixture into the simmering liquid until fully dissolved.
Add the drained rice cakes and fish cakes (if using), stirring to coat evenly in the sauce.
Simmer for 10–15 minutes, stirring occasionally, until the sauce thickens and the rice cakes are soft and chewy.
Remove from heat and divide into bowls; sprinkle the green parts of the spring onions over the top before serving.