Begin by grating your starchy potatoes using a box grater or food processor for fine shreds. Soak the grated potatoes in a bowl of cold salted water for 15 minutes; this helps to remove excess starch. After soaking, drain well and pat them dry with a clean kitchen towel to ensure a crispy potato crust.

In a skillet, heat a tablespoon of olive oil over medium heat. Add chopped green onion and sauté for about 2 minutes until softened. Next, stir in the baby spinach, dried oregano, and a pinch of salt, cooking until the spinach wilts. Finally, add sliced champignon mushrooms and minced garlic, cooking until golden and the liquid has evaporated, approximately 5 minutes.

Drain the soaked potatoes and squeeze out any excess moisture. In a large mixing bowl, combine the potatoes with shredded mozzarella cheese, beaten eggs, chopped parsley, and additional seasoning. Mix thoroughly until everything is well incorporated.

Preheat your oven to 200°C (390°F). Grease a 9-inch round baking dish with olive oil. Press the potato mixture evenly into the bottom and up the sides of the dish to form a sturdy crust. Bake in the preheated oven for 35-40 minutes or until the crust turns golden brown and crispy.

Once the crust is golden, remove it from the oven and let it cool slightly. Spoon the sautéed spinach and mushroom mixture evenly into the cooked potato crust. Sprinkle additional mozzarella and Parmesan cheese on top.

Return the assembled Crispy Grated Potato Pie to the oven and bake for an additional 10-15 minutes until the cheese melts and bubbles.

After baking, allow the pie to cool for about 5 minutes. Garnish with freshly chopped parsley and a sprinkle of Parmesan before serving.