Soak the kombu in 1 liter of water for at least 5 hours. If possible, leave it overnight.
Remove the sand and salt from the manila clams in ahead of time. Please refer to the following recipe for that.
Add the clams into the pot with the kombu, and cook over medium heat.
Remove the kombu right before the water starts to boil. When it's about to boil, turn down the heat. Remove the scum that floats to the surface.
Transfer the clams which have opened into a tray, and remove the shells which comes off from their flesh.
Set aside 5 pieces of clam flesh for dashi. Keep the heat low.
Dissolve in the miso, and return the clams into the pot right before you're ready to eat.
Serve in a bowl, sprinkle with Japanese leeks, and done!