Boil the potatoes until tender, then drain and set aside.
Sauté onion until translucent, then add spinach and cook until wilted. Remove from heat.
Preheat oven, grease casserole dish, and prepare egg mixture.
Layer potatoes, spinach mixture, and egg mixture in the casserole dish. Top with shredded mozzarella.
Bake in preheated oven until golden brown and eggs have set.
Allow to cool slightly before serving. Enjoy!