Start by bringing 4 cups of dashi stock to a gentle simmer in a medium saucepan over medium heat. If using instant dashi, follow package instructions to ensure proper concentration. A gentle simmer, not a boil, is key to preserving the delicate umami.
Place the dried wakame in a small bowl and cover it with warm water. Let it soak for about 5 minutes. It will expand significantly—up to 5 times its original size. Once plumped, drain off any excess water and set the seaweed aside.
Gently add the cubed silken tofu and rehydrated wakame to the simmering dashi. Stir carefully so the tofu doesn’t break apart. Allow everything to warm through for about 2 to 3 minutes until the tofu is heated but not boiling.
Scoop the miso paste into a ladle or small bowl. Ladle in a bit of the hot dashi and mix until the miso completely dissolves into a smooth liquid. Pour this mixture back into the saucepan. Stir gently to distribute the miso evenly. Never boil the soup after adding miso—it will lose its complex flavor and health benefits.
Finally, add the thinly sliced green onions to the soup. Stir gently and remove the saucepan from heat immediately. The residual heat will slightly soften the onions while keeping their fresh flavor intact. Serve hot in small bowls and enjoy.