In a large sauce pan, saute onions in butter. Add spinach and water and cook spinach until spinach is wilted and soft. Add flour and cook for about 1 minute. Slowly add milk and stir until smooth. Add chicken base and continue stirring constantly until soup comes to a boil.
With a immersion hand blender, puree soup until spinach is well blended.
If you would like your soup a little thicker, in a separate small bowl mix together 3 Tablespoons of corn starch to 4 Tablespoons of cold water. Slowly add cornstarch mixture as needed to boiling soup stirring constantly until soup is thickened to desired consistency.
I garnished with some Parmesan cheese.
