- 1 pound (450g) small asari (Manila clams), cockles, or littleneck clams (about 2 dozen Manila clams or cockles, or 8 littlenecks)
- Cold water
- Kosher salt
- 2/3 ounce dried kombu (20g; about one 6- by 6-inch piece), rinsed in cold water
- 3 tablespoons (50g) mild miso, such as shiro (white) miso
- 1 tablespoon (15g) dark miso, such as aka (red) miso
- Mitsuba or watercress leaves, for garnish
- Cooked morel mushrooms (dried or fresh)
- Wild garlic chives
- Rinse clams under cold running water. Fill a large bowl with cold water, season with salt until water is salty like the sea (about 3% salt by weight), and add clams. Let stand for 30 minutes. Lift clams from purging water and discard water. Rinse out bowl. Repeat as many times as necessary until no sand or grit collects in bottom of bowl (usually between 1 and 3 times).
- In a medium saucepan, combine kombu and clams with 3 1/2 cups (830ml) cold water and bring to a bare simmer over medium heat, about 8 minutes. Remove kombu as soon as bare simmer begins and discard or reserve for another use. (Kombu can be thinly sliced and tossed into a salad.)
- Continue simmering, using tongs to transfer clams as they open to a large heatproof bowl, until all clams have opened. Skim any foam from the surface of the dashi. Return dashi to a clean medium saucepan and keep warm over low heat.
- In a small bowl, whisk both misos with just enough dashi to form a thin paste. Off heat, whisk miso paste into dashi in saucepan.
- Divide clams among warm bowls and spoon miso soup and morel mushrooms over them. Garnish with mitsuba or watercress leaves, wild garlic chives, shave truffles and serve right away.