Heat the sesame oil in a large pot over medium heat. Add the sliced onion and minced garlic, then cook for 3 minutes until the onion softens.
Stir in the gochujang and cook for 1 minute to release its aroma.
Pour in the fish stock or water and add the soy sauce. Bring the mixture to a simmer.
Add the sliced daikon radish and cook for 8 minutes until it begins to soften.
Gently place the tofu cubes into the pot and simmer for 3 minutes.
Add the fish pieces and cook for 5 minutes until the fish turns opaque and flakes easily.
Taste and adjust with salt and black pepper if needed. Remove from heat and sprinkle with sliced green onions before serving.