I haven't had miso soup in ages, but I ran across several recipes for it lately, so I had to make some. I ended up adapting a recipe from Mark Bittman's "How to Cook Everything," which is a great reference and recipe book.
Miso Soup with Shiitakes and Spinach (adapted from Mark Bittman's "How to Cook Everything")
2 cups chicken or veggie broth
1 1/2 Tbsp miso
4 oz shiitake mushrooms, sliced
5 oz spinach, roughly chopped
2 green onions, sliced
Bring broth to a boil, then reduce heat to low. Saute mushrooms until tender and set aside. Remove a small amount of broth and mix with miso until it's smooth. Add this mixture, mushrooms, spinach, and green onions to remaining broth. Let sit for a minute before serving. Serves 4.
I didn't like the smell while this was cooking, but I was pleasantly surprised that the taste was delicious. I used a dark miso that I bought before I knew the difference between the types, and I would have probably preferred a light miso. The mushrooms and spinach make it a fairly hearty (but extremely low calorie) soup.