Cook onion and garlic in butter or margarine in a large saucepan over medium heat for 3 to 5 minutes or until onion is tender. Stir in flour and cook for 2 minutes. Carefully stir or whisk in broth. Cook and stir or whisk about 5 minutes or until thoroughly combined and slightly thickened.
Add half-and-half or light cream, lemon peel, white pepper, nutmeg, red pepper, spinach, and watercress; bring to a gentle boil. Reduce heat, cover, and simmer about 10 minutes or until spinach and watercress are tender.
Transfer to a blender container. Cover and blend mixture until smooth. Return to saucepan and rewarm, if necessary. Season to taste with salt, if desired. Makes 6 to 8 (1-cup) servings.