Combine first 5 ingredients in a heavy saucepan over high heat. Cover and bring to a boil. Reduce heat to low and simmer 30 minutes. Using a slotted spoon, transfer vegetables to a food processor or blender. Add a small amount of cooking liquid and purée vegetables. Return purée to saucepan with stock. Add nutmeg and salt and pepper to taste. Place spinach in a food processor or blender. Add 1 cup of soup to spinach and purée. Stir spinach purée into soup and serve hot with croutons.