Preheat your oven to 350F
In a large skillet, heat butter on medium low heat. Add minced garlic to it and stir for about a minute.
Add in your spinach and stir frequently so that the garlic is evenly distributed. It will take a couple of minutes until all of the spinach leaves are wilted. Once the leaves are wilted and it is easier to stir, add in your salt. Cook for at least ten minutes on low heat so that the moisture evaporates.
While the spinach is cooking, make your egg mixture by adding your eggs to a liquid measuring cup. Add the milk to the liquid measuring cup on top of the eggs until your total liquid measures 1.5 cups. Add to a bowl and whisk until combined. Add 1/8 tsp of black pepper if you would like to.
Turn off the heat on the spinach. Remove your pie crust from the freezer and remove it from the packaging. Lay it on a baking sheet. Add enough shredded mozzarella to the bottom of the crust so that there is a light coating (about 1/2 cup)
Add spinach mixture on top of cheese, followed by the feta cheese. Crumble the feta cheese and distribute evenly. Pour the egg mixture on top. Be careful not to over fill the crust. You should stop pouring when the mixture gets close to the top of the sides. It is ok if you have some egg mixture leftover. You can use that to make some scrambled eggs if you want.
Gently evenly distribute the spinach and feta around inside the quiche. Add a light layer of shredded mozzarella cheese to the top. Bake for 48-50 minutes. Let it cool slightly before cutting into it.