In a large heavy pot over medium heat, warm the olive oil. Add the onion and sauté until translucent, about 5 minutes. Add the garlic and spinach, season with salt, and cook, stirring often, until the spinach is wilted, about 5 minutes. Use a slotted spoon to transfer the mixture to a bowl.
In a medium bowl, stir together the egg, ricotta, feta, and Parmesan. Add the spinach, season with pepper, and stir to combine.
Heat the oven to 375 degrees. On a sheet of parchment paper, roll out a pie crust to a 12-inch round. Place the dough, with the paper, on a baking sheet.
Mound the filling in the center of the dough. Smooth it out evenly, leaving a 1 1/4-inch border. Fold the dough edges over the filling to form a crust. Sprinkle on 1 tbsp. of Parmesan, then drizzle with 1 tbsp. olive oil.
Bake the pie on the center rack until the filling and crust are golden brown, about 45 minutes. Let cool at least 15 minutes before serving.