In 3 quart saucepan, saute onion in butter until soft but not brown. Mix in flour, salt, tarragon, nutmeg and cayenne. Cook until bubbly. Mix in spinach, then broth. Bring to a boil. Reduce heat and simmer, uncovered, 15 minutes.
Place in blender, about 1/3 at a time (I used a hand blender right in the pot) and puree. Return to saucepan. Add yogurt, whisking until smooth and blended. Heat just until steaming. (Do not boil.) Add salt, if needed. Serve with a lemon slice on top of each serving. Contributed to the echo by: Ellen Cleary