Boil Dashi. (You can follow our Dashi recipe or use a Dashi packet.) Slice green onions very thinly. Cut tofu into 1/2" cubes and add to Dashi.
Reduce heat to low and dissolve miso paste in the Dashi. Take care not to boil.
Add green onions and remove from heat.
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Notes
Would you like to make your Miso Soup into an instant meal? Drop an egg in the Miso Soup and cook for 2 minutes. Put some steamed rice in a large bowl, and pour Miso Soup with egg over the rice. That's the easiest and tastiest lunch ever!