Directionsmix cornstarch and 1/4 c water, set aside.
bring stock and remaining water to a boil in a saucepan.
reduce heat and add zest, salt, pepper and lemon juice.
whisk in cornstarch mixture and stir 2-3 minutes, simmering.
add egg in a slow stream, stirring with a spoon.
add spinach return to boil, remove from heat.
serve with grated parmesan cheese as a garnish.