Preheat the oven to 400˚F.
Crust: In a medium bowl, beat together the oil and water. Add in the flours and salt, and mix in until fully incorporated.
Lightly spray a pie pan, and press the dough into the pan. Bake in the preheated oven for 15-20 minutes.
Filling: Meanwhile, in a medium skillet, heat the butter on medium heat. Saute the garlic and onion until fragrant and onions are transparent. Stir in the spinach, feta and cheddar cheese, and mix until fully combined. Pour over the pie crust.
In a medium bowl, beat together the eggs and the milk. Season with salt and pepper to taste. Pour over top the spinach mixture.
Turn the oven down to 375˚F, and bake for 35-45 minutes.