Soak the brisket in cold water for 1 hour to draw out the blood. Discard the bloody water.
In a pot, add the brisket, cold water, onion, garlic, dashida, and white parts of green onion. The dashida will clump up- it's ok, just mix it.
Bring to a boil uncovered, then cover and simmer on medium low heat for 1 to 1 1/2 hours. Skim off any impurities that float to the surface.
Meanwhile, make the jidan (egg garnish). Separate the egg whites from the yolk. Season the egg yolk with a pinch of salt and lightly whip it.
On a non stick pan without oil (or very lightly oiled), spread the yolk evenly and thinly over low heat. Then turn off the heat and flip it over once it has set. It should not be browned or have many air bubbles.
Roll up the jidan and cut into thin slices. Set aside.
When the broth is done, turn off the heat and remove the brisket. Set it aside to cool and shred.
Strain the broth and discard the aromatics.
Add all the beef seasoning ingredients and mix well. Set aside.
Season the broth with fish sauce and soup soy sauce.
Add sea salt and black pepper to taste. Careful not to add too much sea salt- the beef is also seasoned with sea salt so keep that in mind.
Add the dumplings- I'm using frozen dumplings which take 7-8 minutes to cook, so I'm adding them before the rice cakes which will only take 3-4 minutes.
Rinse the rice cakes then add them to the soup. Cook for 3 minutes. The mandu and rice cakes will float once they are ready. If you are using frozen rice cakes, you will need to soak them in water for 5-10 minutes before adding to the soup.
Add 1 tsp sesame oil, chopped green onions, and egg whites. Stir then turn off the heat.
Serve immediately and top with the seasoned brisket, jidan (egg yolk strips), and roasted seaweed strips. Note: do not wait too long to serve. The rice cakes will absorb too much broth and get very soggy.