Donabe Seafood Soup with Udon Noodles

Name: Donabe Seafood Soup with Udon Noodles
Ingredients: One 4-inch square kombu (about 3 ounces), wiped with a damp paper towel to remove any sand; 1 cup bonito flakes; 1 tablespoon vegetable oil; One 1-inch piece ginger, sliced into thin matchsticks; 1 scallion, trimmed and cut into 2-inch pieces; 4 medium leaves napa cabbage, cut into 2-inch pieces (about 4 cups); 1/4 cup mirin, plus more for seasoning; 3 tablespoons light soy sauce, plus more for seasoning; 2 packages fresh udon noodles (about 15 ounces), each package broken into quarters; 6 ounces assorted mushrooms, such as enoki, oyster and shiitake, trimmed, large mushrooms sliced into bite-size pieces on the bias; One 6-ounce piece snapper fillet, cut into 2-inch-wide slices; 4 shell-on large shrimp (about 4 ounces), deveined; 4 littleneck clams, scrubbed; 1 small bunch flat garlic chives (about 4 ounces), trimmed and cut into thirds; 1 teaspoon toasted sesame oil