Bring a large pot of water to a boil. Meanwhile, in a large Dutch oven over medium-high, heat the oil until barely smoking. Add the cabbage, onion and ginger; cook, stirring occasionally, until the vegetables are softened and beginning to brown, 4 to 5 minutes. Add the garlic, gochujang and fish sauce; cook, scraping up any browned bits, until fragrant, about 1 minute. Stir in the clams and add the dashi. Cover and bring to a boil, then reduce to medium and cook, stirring occasionally, until the clams open, about 5 minutes; remove and discard any that do not open.
While the clams cook, add the noodles to the boiling water. Cook, stirring occasionally, until tender. Drain in a colander, rinse briefly under warm water and drain again. Divide the noodles among 4 serving bowls.
Season the cod and scallops (if using) with salt and pepper, then place in a layer on top of the clams; do not stir. Cover and cook without disturbing for 2 minutes. Remove the pot from the heat and let stand, covered, until the fish is opaque throughout and flaky, about 6 minutes, scattering the scallops (if using) in the pot and re-covering after 2 minutes of standing.
Ladle the soup over the noodles, evenly dividing the fish and shellfish. Sprinkle with the scallions and drizzle with sesame oil.
