Preheat your 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 sauté for 3-4 minutes until translucent. Add the minced garlic and cook for an additional minute.

Stir in the chopped spinach and cook until wilted, about 2-3 minutes. Remove from heat and let cool slightly.

In a large mixing bowl, combine the cooked potatoes, sautéed spinach mixture, ricotta cheese, mozzarella cheese, egg, salt, black pepper, and nutmeg. Mix until well combined.

Pour the filling into the pre-made pie crust, spreading it evenly.

Bake in the preheated oven for 30-35 minutes, or until the pie is golden brown and set.

Allow the pie to cool for about 10 minutes before slicing and serving.