Whisk eggs, Greek yogurt, mustard, salt, and pepper together. Set aside.
Meanwhile, slice the apples and potatoes. You want them uniformly sliced at about 1/16-inch thick.
Arrange the apples and potatoes in a circle along the baking dish. Continue to layer until you fill the container.
Pour in the egg mixture.
Top with cheese.
Bake for 50-60 minutes, or until just set and lightly brown. You want the egg to still juggle slightly, but not be liquid.
Serve immediately or chilled. Quiche is a great recipe to make ahead of time because it keeps well and is just as good cold. If you do reheat it, though, I would recommend heating in the oven. Oftentim
Tips, Tricks, & Notes
I have an entire article on The Fundamentals of Crustless Quiche with more tips and tricks on whether or not you want a crust, whether or not you need dairy or cheese, the type of pan to use, and a recommendation of toppings.