Preheat oven to 400°F (200°C).
Boil potatoes until fork tender. Drain and allow them to steam dry.
Press potatoes into a greased 9-inch pie dish to form a crust.
Brush with olive oil and
Bake for 40–45 minutes until golden.
While the crust bakes, slowly caramelize the onions until they're deep golden brown and sweet.
Reduce oven temperature to 350°F (175°C).
Whisk together eggs, milk, heavy cream, garlic powder, onion powder, and salt.
Stir in caramelized onions, chopped spinach, and feta.
Pour into the baked potato crust.
Top with mozzarella.
Bake for 35 minutes, until the center is just set.
Rest for 15 minutes before slicing.