Preheat the oven to 350ºF. Thaw and squeeze as much moisture out of the spinach as possible.

Rinse the mushrooms, then slice thinly. Mince the garlic.

Add the mushrooms, garlic, salt, and ½ Tbsp cooking oil to a skillet. Sauté the mushrooms over medium heat until all moisture has evaporated.

Brush ½ Tbsp cooking oil inside a 9-inch pie plate. Layer the mushrooms, spinach, and crumbled feta into the plate.

In a large bowl, whisk together the eggs, Parmesan, pepper, and milk.

Pour the egg mixture into the pie plate over the spinach, mushrooms, and feta. Top with shredded mozzarella.

Bake the quiche in the preheated oven for about 50 minutes, or until golden brown and the internal temperature reaches 160ºF.

Slice and enjoy.