In a food processor, process the flour, salt, ice water, egg and pieces of butter until fully combined. You may need to stir it a few times to really break up the butter. Feel free to add a little water if it is not combining enough.
Transfer to a clean baking surface sprinkled with flour, and press until a solid dough ball forms. Smush the dough ball down and begin to roll it out with a rolling pin. You want the size of the dough circle to be about 1 1/2 inches wider than your pie or tart dish.
Spray the tart dish with cooking spray and transfer the dough to the tart or pie dish. Shape the dough into the tart, pressing down the sides and discarding the extra dough. Cover with plastic wrap and refrigerate until ready to use!
Make The Quiche Filling:
Pre-heat the oven to 375 degrees F.
In a large bowl, mix together the egg whites, garlic powder, chopped onion, cherry tomatoes and shiitake mushrooms. Toss in about 1/2 tsp of dried thyme and a little ground black pepper. Grate in about 1/2 a cup of gruyere cheese and mix well.
Pour the egg mixture into the quiche crust, and level with a spoon. With your hands, crumble the corned beef hash evenly over the top of the quiche. Make sure to keep it on the top and to NOT mix it, because we need it to get crispy!
Bake your quiche in the oven at 375 degrees F for about 45 minutes, until the edges of the quiche crust are golden. Allow it to rest for at least 10 minutes before removing from your tart or pie tin and cutting. Enjoy!
Notes
TIPS:To make ahead, make the entire recipe, and cover the quiche with plastic wrap and place it in the refrigerator for no longer than 24 hours.
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