Prepare the Bowls: Roughly chop or shred the clean, dry spinach and divide it evenly among four serving bowls.
Make the Miso Broth: In a medium saucepan, bring the 6 cups (1440 ml) of water to a boil. Ladle about 1 cup (240 ml) of the hot water into a small bowl and whisk in the miso paste until it is completely smooth and dissolved.
Pour this miso slurry back into the saucepan and stir to combine.
Season and Simmer: Stir the toasted sesame oil, tamari, and a few grinds of black pepper into the broth.
Bring the soup to a gentle simmer (do not let it come to a rolling boil).
Add the diced tofu and heat for 1-2 minutes.
Assemble and Serve: Carefully ladle the boiling hot soup directly over the raw spinach in each bowl. The heat will wilt the spinach perfectly.
Garnish generously with sliced scallions and serve immediately.
Don't have spinach? Finely shredded bok choy, napa cabbage, or kale (stems removed) work beautifully as a substitute.