Rinse the kombu and dried shiitake mushrooms under cold running water, and then add to a medium stockpot.
Add 12 cups water, and allow to soak for 1 hour.
After soaking, place the pot over medium heat, and allow the stock to come up to just a simmer.
Fish out the kombu, increase the heat to high, and bring to a boil.
Once boiling, reduce the heat to low and add the bonito flakes.
Simmer for 20 minutes, stirring occasionally.
Strain the resulting dashi stock into a medium saucepan to make your soup (reserving the shiitake mushrooms), and place it over medium low heat.
Cut the tofu into ½-inch cubes and set aside.
Whisk the miso into the stock until fully incorporated.
Add the tofu, spinach, and scallions.
Slice a couple of your shiitake and add those to the pot as well.
Simmer for another couple minutes and serve.
