Get your salmon stock or dashi warm in a pot. Cook your soba noodles according to the instructions on the package. Drain and set some in each person's bowl.
Add some salmon to each bowl, along with some green onions.
Mix the miso with the stock -- you can double the amount of miso if you'd like -- and pour into each person's bowl. Serve at once.
Notes
I use meat scraps from making the salmon stock for this recipe, but you can use diced salmon if you want. Any type of salmon, trout or char is a good choice here, as is whitefish, sablefish or any of the jacks or small tuna.
Keys to Success
Since this soup is so simple, your ingredients will make or break it. Get soba from Japan, really good miso, and make the broth yourself and you will be amazed.
As I mention above, I use red miso on cold days, white on warm ones. Red is stronger flavored. How much you use is variable. Use the amounts in my recipe as a guide, not dogma.
Alternatives to green onions would be scallions, chives, chrysanthemum leaves, and shiso.