Cut the kimchi and onion into bite-sized pieces.
Heat oil in a pot and stir-fry the kimchi and onion.
Drain the oil from the canned tuna, then add the tuna to the pot.
Add red pepper powder and minced garlic, and stir-fry together.
Pour in water and bring to a boil.
Cut the tofu into bite-sized pieces and add to the pot.
Season with soy sauce for soup and salt.
Simmer over medium heat for 10–15 minutes.
Slice the green onion and add it at the end, then bring it to a gentle boil one more time.