Bring the stock or water to a boil in a saucepan. Add the alphabet pasta and boil gently until done, about 6 minutes, or according to the package directions.
Cut the tofu into small cubes, and thinly slice the green onions, and set aside.
When the pasta is tender, take the soup off the heat. Stir in the miso paste, and then add the tofu.
Serve in small bowls, garnished with the sliced green onions.