In a large frying pan over high heat, combine onion and 1/4 cup water. Stir often until liquid evaporates and onion starts to brown. Add 1/4 cup chicken broth and stir to release browned bits in pan; boil and stir often until pan is dry.
Remove from heat and mix in flour. Gradually stir in remaining 1 cup broth and 1/2 cup sour cream, stirring until smooth. Add nutmeg and lemon zest and stir over high heat until boiling vigorously. Stir and boil about 3 minutes, then remove from heat and stir in spinach.
For creamier spinach, purée 1/4 to 1/2 the spinach mixture in a blender or food processor; stop and start motor frequently, and push mixture down into blade. Return puréed spinach to pan.
Add remaining 1/2 cup sour cream and the salt and pepper to taste.
Make ahead: Cover and chill up to 1 day. Stir creamed spinach over medium-high heat until hot.
Note: Nutritional analysis is per serving.