Melt butter in a large saucepan over medium heat. Add the onion, and cook until tender, stirring occasionally. Stir in the flour, salt, tarragon, nutmeg and cayenne, and heat until fragrant. Stir in spinach (undrained) and chicken broth. Bring to a boil, then reduce heat to low, and simmer for about 15 minutes.
Remove the soup from the heat, and puree in a food processor or blender in batches. Return to the saucepan, and whisk in yogurt. Heat through, but do not boil. Taste and adjust seasonings if necessary. Ladle into bowls, and float a lemon slice on top of each serving.
