How to Make It
In a large saucepan, bring 4 cups of water to a boil over high heat. Add the clams, cover and boil until they open, about 4 minutes. Using tongs, transfer the clams to a large plate. Remove the clams from the shells and reserve; discard the shells.
Slowly pour the clam broth into a large bowl, stopping when you reach the grit on the bottom. Rinse the pan and put it back on the stove. Pour the clam broth back into the saucepan and whisk in the miso until smooth. Bring the soup to a simmer over moderate heat. Add the tofu and cook for 4 minutes.
Return the clams to the soup to heat through. Sprinkle the scallions on top and serve immediately.