In a large pot, boil 6 cups of broth and onions. Lower heat, place lid on pot, and simmer 20 minutes or until onions are tender. Take onions out of stock using a slotted spoon and set aside; save broth.
In a medium pot, combine spinach and 3 cups of broth and let boil. Lower heat, place lid on pot, and simmer 10 minutes. Strain, pressing on spinach to remove any juices. Throw away spinach or save for another use.
Add stock from onions and spinach into a large pot and heat. Sprinkle with salt and pepper. When ready to serve, top soup with onions and green onions.