Add the clams, cover, and simmer gently until the clams open, about 5 minutes. Remove from the heat.
Measure the miso into a small bowl and add a ladleful of the hot clam broth. Knead the miso with the back of a spoon until it dissolves into the broth, then pour the liquid back into the pan along with the cream. Season with salt to taste and a generous amount of pepper and stir in the scallion greens. Divide among 4 serving bowls and garnish as you see fit.