Soak the rice cakes in warm water for 30 minutes if they're hard.
In a pot, combine the water, gochujang, gochugaru, soy sauce, and sugar.
Bring the mixture to a boil over medium heat, stirring until smooth.
Add the soaked rice cakes to the pot.
Cook for about 5 minutes, stirring gently to coat the rice cakes.
Add the fish cakes and sliced onion to the pot.
Continue to cook for another 5 minutes until the sauce thickens.
Stir in the chopped green onions.
Remove from heat and let it cool slightly.
Serve hot, garnished with sesame seeds.