Rinse clams under cold water, discarding any that are open or cracked.
In a medium pot, bring dashi to a gentle simmer over medium heat.
Add clams and sake (if using). Cover and cook until clams open, about 4-6 minutes. Discard any unopened clams.
Reduce heat to low. Place miso paste in a small ladle, dip ladle into broth, and whisk until miso dissolves. Add tofu and gently heat through, about 1 minute. Do not boil.
Ladle into bowls, garnish with green onion, and serve immediately.
Tips
Use white miso for a milder flavor; red miso can be substituted for a stronger taste.
Clams can be replaced with small littleneck clams; adjust cooking time as needed.
Leftover soup can be refrigerated for up to 1 day, but reheat gently to avoid toughening clams.