Preparation Stir together cornstarch and 1/4 cup water until smooth. Bring broth and remaining 3/4 cup water to a boil in a 2- to 3-quart heavy saucepan over high heat, then reduce heat to moderately high and add zest and salt. Stir cornstarch mixture and add to soup, whisking until smooth. Simmer, whisking, 2 minutes, then add egg in a slow stream, stirring with a spoon. Stir in spinach and return to a boil, then remove from heat. Season with salt.
Preparation Stir together cornstarch and 1/4 cup water until smooth. Bring broth and remaining 3/4 cup water to a boil in a 2- to 3-quart heavy saucepan over high heat, then reduce heat to moderately high and add zest and salt. Stir cornstarch mixture and add to soup, whisking until smooth. Simmer, whisking, 2 minutes, then add egg in a slow stream, stirring with a spoon. Stir in spinach and return to a boil, then remove from heat. Season with salt.