Gather Ingredients: Start by assembling all necessary ingredients for both the dashi and the miso soup, including kombu, katsuobushi, tofu, miso paste, dried wakame, and a green onion.
Prepare the Green Onion: Slice one green onion into thin rounds, setting them aside for later use in the miso soup.
Kombu Dashi Setup: Place 4 cups of water in a saucepan and add the kombu. Let it soak for about 30 minutes if time allows, enhancing the flavor extraction without washing or removing its white umami-rich substance.
Heat Kombu Dashi: Slowly heat the kombu and water on a medium-low setting until it's just about to boil, roughly 10 minutes. This method extracts the maximum umami. Remove the kombu before it boils to avoid a slimy and bitter taste.
Add Bonito Flakes: For a non-vegetarian dashi, introduce katsuobushi to the pot after removing the kombu. Bring it to a brief boil, then simmer for 30 seconds. Turn off the heat and let it sit for about 10 minutes for the flakes to fully infuse the dashi. Strain to achieve a clear broth.
Miso Integration: Heat the prepared dashi in a saucepan until it's near boiling. Mix the miso paste in a ladle with some dashi, ensuring it's fully dissolved without lumps, using either a muddler or a strainer for efficiency.
Tofu Addition: Cut the tofu into ½-inch cubes, ensuring the miso is fully mixed into the dashi before adding them to prevent breaking the delicate tofu pieces.
Wakame and Onion: Just before you're ready to serve, add the rehydrated wakame seaweed and sliced green onions to the soup. This step ensures their freshness and vibrant color are preserved.