Peel the nagaimo, and slice into about 5 mm quarter-rounds. (I cut into quarter-rounds, but using your preferred way of cutting is fine.)
When you add the nagaimo into a boiling dashi stock, it will start to bubble, so turn down the heat and skim off the scum.
Once the nagaimo turns translucent, dissolve in miso and bring to a boil. Turn off the heat, and add the dried wakame seaweed.
(For wakame, you can use salted or fresh ones. In that case, add at Step 3 before dissolving in the miso.)
Story Behind this Recipe
I bought one whole nagaimo, and this is one of the dishes I made in an attempt to use it all up.
Be careful when peeling the nagaimo, since your hands could slide off due to its sliminess. The salt content of the miso depends on the brand, so adjust its amount to your taste. I used wakame seaweed, but green onions, myoga ginger, okra, snow peas, etc. makes it delicious as well so give it a try.