In a medium bowl, combine sliced beef sirloin, soy sauce, sesame oil, minced garlic, grated ginger, and a sprinkle of black pepper. Mix well and let it marinate for at least 20 minutes.

While the beef is marinating, bring a large pot of water to a boil, add the Korean ramen noodles, and cook according to package instructions (about 3-4 minutes). Drain and toss with a little oil.

Heat a large skillet over medium-high heat, add vegetable oil, and sear the marinated beef for 3-4 minutes on each side. Transfer to a plate and let it rest.

In the same skillet, lower the heat, add chopped onion, and sauté for 2-3 minutes. Then add beef broth, gochugaru, and gochujang, and simmer for about 5 minutes.

Stir the cooked ramen noodles into the broth, pour in the heavy cream, and fold in kimchi and half of the chopped green onions. Simmer for another 2-3 minutes.

Ladle the ramen into bowls, top with seared beef slices, remaining green onions, and a sprinkle of sesame seeds. Serve hot.