Place the spinach, potatoes, water, chicken backs (or wings), and onion in a large pot over medium-high heat. Cover the pot and bring to a boil. Add salt, pepper, and oregano. Reduce heat to medium-low, and simmer covered until vegetables are tender, stirring occasionally.
Add the cream, and cook over low heat for a minute. Remove the chicken before serving. Serve with lemon juice on the side.
Note: If it's too thick, add milk. Add more heavy cream for richer soup.