Preheat the oven to 375°F (190°C).
In a large pot, bring water to a boil and add the diced potatoes. Cook for about 10-12 minutes or until tender. Drain and set aside.
In a skillet, heat olive oil over medium heat. Add the chopped onion and garlic, sautéing until the onion is translucent, about 5 minutes.
Add the chopped spinach to the skillet and cook until wilted, about 3-4 minutes. Remove from heat.
In a large mixing bowl, combine the cooked potatoes, sautéed spinach mixture, ricotta cheese, mozzarella cheese, egg, salt, pepper, and nutmeg. Mix until well combined.
Roll out the pie crust and place it in a 9-inch pie dish. Pour the spinach and potato filling into the crust, spreading it evenly.
Bake in the preheated oven for 30-35 minutes or until the crust is golden brown and the filling is set.
Allow the pie to cool for 10 minutes before slicing and serving.