Preheat oven to 375°F (190°C).
Combine almond flour, tapioca starch, sea salt, melted butter, and one egg in a mixing bowl until a uniform dough forms.
Press the dough firmly into a 9-inch pie dish, ensuring even thickness across the bottom and up the sides.
Par-bake the crust for 10 minutes to set the structure, then remove from oven.
In a separate bowl, whisk the remaining four eggs with heavy cream, milk, kosher salt, pepper, and nutmeg until emulsified.
Distribute the prepared spinach and crumbled feta cheese evenly over the par-baked crust.
Pour the egg mixture over the fillings, ensuring even distribution.
Bake for 30 to 35 minutes until the custard is set but slightly jiggly in the center and the edges are golden brown.
Allow to rest for 15 minutes at room temperature before slicing to allow the protein structure to stabilize.