Add sesame oil to a pot over medium heat. Add onion, garlic, kimchi, and sugar, and cook for about 3-4 minutes, stirring often to ensure even cooking. Add rice water and bring to a boil.
When the stew starts boiling, add kimchi juice, Gochugaru, and Doenjang (or soy sauce). Cook for about 8 minutes, or to your desired consistency.
Add tuna and tofu and cook for another 2 minutes, and season with salt to taste, and add in Dashida if using. Garnish to your preference and enjoy!