Heat six tablespoons butter in a 2 quart soup pot and cook onion until tender.
Add chicken stock and diced potatoes. Cook over medium heat for 30 minutes. If you
like, you may allow potatoes to disintegrate or you may crush them as they cook.
Add minced garlic, onions and spinach. Simmer for another 10 minutes.
and simmer over low heat.
In a small bowl, add 1/2 cup of soup to 2 egg yolks, in a stream while whisking,
taking care not to curdle the yolks by adding the hot soup too quickly. Add whisked
mixture to the soup pot, stirring constantly until the soup is thickened.