Few sprigs Japanese mitsuba or chervil, to garnish
1) Place the clams in a basin, scrub with a brush to clean, then rinse. Soak the clams in cold water for 10 minutes. Drain, rinse well and set aside.
2) Meanwhile, place the water in a large saucepan and bring to a boil. Add the mushrooms, dashi powder and nori, reduce heat to low and simmer for 15 minutes covered. Increase heat to medium, add clams and cook for 2-3 minutes covered or until shells open slightly, discarding any that do not open. Remove pan from heat and discard the mushrooms.
3) Divide clams into 4 serving bowls using a slotted spoon. Place miso paste into another small bowl and stir in a little of the soup stock until the paste is of pouring consistency. Gradually stir the miso mixture into the soup stock.
4) To serve, pour miso soup over the top of the clams and garnish with sprigs of mitshuba or chervil.