01 - Combine sliced beef with soy sauce, sesame oil, cornstarch, and black pepper. Set aside at room temperature for at least 10 minutes.
02 - Bring a pot of water to a gentle boil. Lower in the eggs and cook for 6 to 7 minutes for soft yolks. Transfer eggs to ice water to cool, then peel and halve.
03 - Heat vegetable oil in a large pot over medium heat. Sauté onion, garlic, and ginger until fragrant, about 2 to 3 minutes. Stir in chili paste and miso paste, cooking for 1 minute.
04 - Add stock, soy sauce, mirin (if using), rice vinegar, and sugar. Bring to a gentle simmer and cook for 10 minutes to develop flavors.
05 - Add marinated beef and sliced mushrooms to the simmering broth. Cook for 5 to 7 minutes until beef is just cooked and mushrooms are tender. Add spinach or bok choy and cook for 1 to 2 minutes more until wilted.
06 - Cook ramen noodles according to package instructions. Drain and rinse briefly under hot water to prevent sticking.
07 - Divide noodles evenly among bowls. Ladle hot broth with beef, mushrooms, and greens over the noodles. Top each serving with halved soft-boiled eggs, spring onions, toasted sesame seeds, nori strips, and sprinkle with chili flakes as desired.
08 - Serve immediately while hot for optimal flavor and texture.