Soak seaweed in cold water for 10-15 mins, then drain and slice.
Heat oil and sesame oil in a soup pot over medium-low heat. Add minced onion and stir-fry for 30 seconds. Add the beef and cook until no longer pink. Add the seaweed, garlic, and 1 tablespoon of Korean soup soy sauce; cook for 3 minutes
Pour in rice water (or plain water) and stir. Cover with a lid and simmer for 10 minutes or until the seaweed is soft and tender.
Season the soup with the remaining Korean soup soy sauce, Korean tuna sauce (if using), salt, and 1 tsp sesame oil. Serve hot with rice and kimchi.