Make Dashi broth according to Dashi recipe. Or you can use an instant Dashi.
Cut green onion finely, and slice 1/2 brown onion thinly. Dice tofu.
Boil brown onion in the dashi broth for 5 minutes. Add tofu and simmer for a few minutes.
Dissolve miso paste in the soup. Serve in a bowl and top with green onion.