Cut off the top and the root end of the onion. Peel off the outer layers of skin. Rinse. Cut the onion in half. Cut onion into large dice. Peel the potatoes. Rinse. Cut into cubes. Trim the roots off the spinach. Wash thoroughly. Separate the leaves. Cut the stems into short lengths. Put Miso into a mesh strainer.

Boil water in a pot. Add yellow onions in boiling water and cook for 1 minute. Add potatoes cubes and cook until almost done; about 4 minutes. Add spinach and cook for another 1 minute. Turn off heat. Dissolve the Miso into the soup using a spoon. Taste the soup and see if you want to add more Miso. Reheat and serve immediately.