1. Rinse spinach and shred with a knife, set aside.
2. Heat 6 tbsp butter in a 2 quart soup pot and cook onion until tender.
3. Add the chicken stock, and diced potatoes.
4. Cook on medium heat for 30 minutes.
5. If you like, you may allow the potatoes to disintegrate or crush them as they cook.
6. Add the minced garlic and spinach, simmer for another 10 minutes.
7. Add the cream. Do not substitute milk or half and half or the soup will be weak.
8. Be sure to heat without boiling.
9. In a small bowl, add 1/2 cup of the soup to the 2 egg yolks in a stream while whisking, taking care not to curdle the yolks by adding the hot soup too quickly.
10. Add the whisked mixture back into the soup pot, stirring constantly until the soup is thickened.
11. Do not boil. Add salt and pepper to taste.
12. Remove from heat and swirl in remaining butter.
13. Serve right away.