Remove pie crust pouches from box; leave at room temperature.
Preheat oven to 450 degrees F (230 degrees C).
Heat olive oil in a skillet over medium heat. Add onion; cook until translucent, about 3 minutes. Add spinach; cook until all moisture has evaporated, about 2 minutes. Remove from heat and let cool.
Combine ricotta cheese, 1 1/2 cups mozzarella cheese, Parmesan cheese, salt, and pepper in a bowl. Add cooled spinach mixture; mix topping well.
Unfold a pie crust onto a nonstick baking sheet; press out fold lines with fingertips. Spread half the topping onto the crust, covering all but 1 inch of the edge. Fold the edge over the topping. Repeat unfolding, spreading, and folding the second crust. Sprinkle the remaining 1/2 cup mozzarella cheese evenly over the 2 pies.
Bake in the preheated oven until the crust is lightly browned and the topping is bubbly, about 20 minutes.