01 - Combine sliced beef with soy sauce, sesame oil, cornstarch, and black pepper in a bowl. Set aside to marinate 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. Add onion, garlic, and ginger; sauté until fragrant, about 2 to 3 minutes.
04 - Stir in chili paste and miso paste; cook for 1 minute. Add stock, soy sauce, mirin (if using), rice vinegar, and sugar. Bring to a gentle simmer and cook for 10 minutes.
05 - Add marinated beef and shiitake mushrooms to the simmering broth. Cook until beef is just done and mushrooms are tender, about 5 to 7 minutes. Add spinach or bok choy and cook until wilted, 1 to 2 minutes.
06 - Cook ramen noodles according to package instructions. Drain and briefly rinse under hot water to remove excess starch.
07 - Divide noodles among bowls. Ladle hot broth with beef, mushrooms, and greens over noodles. Top each bowl with halved soft-boiled eggs, spring onions, toasted sesame seeds, nori strips, and chili flakes to taste.
08 - Serve immediately while hot.