In a large bowl, soak the rice cakes in tap water for 30 minutes.
While the rice cakes are soaking, heat oil in large saucepan over medium heat. When hot but not smoking, add marinated beef and stir fry a few minutes over high heat until it has turned color. With a slotted spoon, remove half the beef into a bowl. Add 7 cups water into the saucepan. Cover and bring to a boil over high heat. When boiling lower heat to a simmer, 20 minutes or so.
While broth is cooking, heat skillet and coat bottom with cooking spray or oil. When hot, pour in enough of the beaten egg to cover bottom of pan. Cook briefly, just until set, and remove to a cutting board. Repeat with the remaining egg, coating bottom of skillet with spray or oil each time, until all the egg is gone. Julienne and set aside.
Cut nori with scissors into thin strips, or crush into pieces with your hands.
Add scallions to the broth. Taste for seasoning and adjust using fish sauce and/or soy sauce. Add the soaked rice cakes and cook 7-8 minutes or just until tender. Do not overcook.
While rice cakes are cooking, heat skillet and coat with a bit of oil. Over high heat, return the beef that was set aside and stir-fry a few minutes or just until heated through.
Ladle broth and rice cakes into bowls, top with beef, julienned egg, and gim/nori strips. Sprinkle with (optional) chopped scallions and sesame seeds. Serve with shichimi togarashi and kimchi on the side.