Drain the clams and save the oil. Snip the chives and halve the nori. Toast the nori for about 20 seconds over a gas flame or under a hot grill, until crisp.
Warm the oil in a medium pot over medium heat. Add the garlic, ginger and shallot and cook 1 to 2 minutes, until the shallot softens. Do not let the garlic brown.
Pour in the stock and bring it to a gentle simmer, about 3 minutes.
Add the mushrooms and cook for 2 to 3 minutes, until tender, then lift them out and set them aside.
Ladle a little hot stock into a small bowl, stir the miso into it until smooth, then pour it back into the pot with the soy sauce. Keep the heat low and do not boil.
In a separate pot of salted boiling water, cook fresh noodles for 1 1/2 to 2 minutes, or follow the package for dried ones, until al dente. Drain well.
Taste the broth and add a pinch of salt only if it needs it.
Divide the noodles between two warm bowls, ladle in 250 to 300 ml of hot broth per bowl, and arrange the mushrooms and clams on top. Finish with chives, nori and a few drops of the reserved clam oil.