Soak and heat the kombu: Place the kombu and water in a pot and soak for at least 20 minutes. You can also soak it overnight in the refrigerator. Place the pot over low to medium heat. When small bubbles begin to form along the bottom of the pot, remove the kombu and continue heating until the water comes to a boil.
Add the bonito flakes: Once the water boils, remove the pot from the heat and add the bonito flakes. Let them steep for 3–5 minutes. Skim off any foam if desired.
Strain the dashi: Pour the broth through a fine-mesh strainer. Gently press the bonito flakes to extract the remaining broth, but don’t squeeze them too hard, as this can make the broth bitter. Return the dashi to the pot.
Prepare the tofu: Cut the tofu into bite-sized cubes. For soft tofu, gently cut it while holding it on the palm of your hand to help prevent it from breaking. You can also cut it on a cutting board if preferred.
Add the tofu and wakame: Add the tofu and dried wakame to the dashi. Once the wakame has rehydrated, turn off the heat.
Add the miso: Place the miso paste in a ladle and gradually dissolve it into the broth. Do not boil the soup after adding the miso, as high heat can diminish its flavor and aroma. Serve warm, topped with sliced green onions if desired.