Prep the seafood. Rinse and pat dry. If using mussels or clams, scrub and debeard.
Slice squid into rings. Season lightly with salt and pepper.
Cook the noodles. Boil according to package directions until just shy of done. Rinse briefly to stop cooking.
Drain and set aside.
Make the chili oil base. Heat neutral oil in a large pot over medium. Add gochugaru and stir for 20–30 seconds until fragrant. Don’t burn it.
Build the aromatics. Add garlic, ginger, and the white parts of the green onions.
Stir 30 seconds. Add onion and cook until slightly softened.
Add pork (if using). Stir-fry until no longer pink. This adds richness and depth to the broth.
Stir in the vegetables. Add cabbage, carrot, mushrooms, and zucchini.
Cook 2–3 minutes until just starting to soften.
Season the base. Add soy sauce, oyster sauce, gochujang (if using), and sugar. Stir well to coat everything.
Pour in the broth. Add 6 cups stock and bring to a rolling boil. Taste and adjust salt and pepper.
Add the seafood. Drop in shrimp and squid first; simmer 1–2 minutes.
Add mussels or clams and cook until shells open. Don’t overcook.
Finish the soup. Stir in sesame oil and the green parts of the green onions. The broth should be bright red and aromatic.
Assemble. Portion noodles into bowls.
Ladle the hot soup and seafood over the noodles. Serve immediately.