In a pot, combine green onions, garlic, potato, nutmeg and broth; bring to a boil over high heat. Cover, reduce heat to medium-low and boil for 5 to 10 minutes or until potatoes are almost tender.
Meanwhile, trim off tough stems from spinach. Stir spinach and peas into soup; cover and boil for about 5 minutes or until spinach is tender.
Use an immersion blender in the pot, or transfer to a blender in batches, and purée until smooth. Return to pot, if necessary. Add milk and heat over low heat, stirring, for 2 to 3 minutes until steaming (do not let boil). Drizzle in lemon juice, stirring constantly; season to taste with pepper.
In a small bowl, stir together yogurt and basil, mint or chives. Ladle soup into warm bowls and add a dollop of herbed yogurt; swirl slightly.