1. Cook noodles as package directs and set aside; prepare broth.
2. Bring kombu and water to a boil; remove pan from heat, add bonito flakes, cover and allow flakes to sink to the bottom.
3. Remove kombu and discard flakes (keep kombu to cook in other dishes).
4. Add soy sauce and mirin and lightly simmer for 1 minute.
5. Add the juice from the grated ginger, if using.
6. Place noodles in bowls and pour hot broth to cover; top with green onions.