Cut kimchi and onion into bite-sized pieces.
Heat cooking oil in a pot, then stir-fry minced garlic, onion, and kimchi.
When the kimchi is cooked, add the tuna and stir-fry together.
Add water, soy sauce for soup, and chili powder, then bring to a boil.
Once boiling, add the soft tofu, cover with a lid, and simmer for 5 more minutes.
Season with salt and pepper to taste.
Slice green onion thinly, garnish the stew, and turn off the heat.