The secret to great miso soup, agree Chowhounds, lies in homemade dashi, the basic stock made from kombu (a seaweed) and bonito flakes. Happily, dashi is easy to make once you’ve got those ingredients.
thew explains miso soup start to finish: Put a piece of kombu in a pot with cold water and slowly heat the water; take the kombu out just before the water boils (this should take at least 10 minutes). Bring the water to a boil and add bonito flakes, then take the pot off the heat. After a minute or two, the flakes will saturate and sink. Strain out the solids. Place a little of the hot dashi in a small bowl and whisk in the miso until it is smooth. Add it back to the pot. Don’t allow the miso soup to boil.
FoodFuser likes to soak some dried shiitakes in hot water and add that to the dashi, along with a splash of fish sauce. sixelagogo recommends using both white and red miso in a 60 percent–to–40 percent ratio. FoodFuser notes that classic garnishes include half-inch cubes of soft tofu and a sprinkle of green onion.
Board Link: What is the Secret to Miso Soup?