Preheat the oven to 400°F.
Peel, core, and thinly slice the apples.
Place them in a medium-size bowl, pour the apple cider over the top, and set aside.
To make the crust mix together the cassava flour, ¼ teaspoon of salt, and 2 tablespoons of sugar in a medium-size bowl.
Cut in the butter until the mixture resembles coarse crumbs.
In a small bowl beat the egg and ¼ cup of milk together.
Make a well in the center of the flour mixture and pour in the egg mixture.
Stir until just combined.
Place the dough for the crust in the refrigerator until it is time to roll it out.
To make the apple filling place a 9-10 inch cast iron skillet or frying pan on the stove on medium heat.
When the pan is hot, add the ½ cup of sugar, water, and the cider from the apples and bring to a boil.
Add the sliced apples to the pan.
Cook about 15 minutes, gently stirring occasionally until the sauce is reduced and starting to turn a golden brown.
While the apples are cooking, generously sprinkle cassava flour on the countertop and roll the refrigerated dough into a circle large enough to fit inside the skillet and cover the apples.
Once the apples are done cooking, it is time to top them with the crust. If you are using an oven-proof pan you can do this directly in the pan. If not, you will need to transfer the apples to a baking dish, such as a pie pan.
Place the rolled out dough carefully over the top of the apples, using a spoon to tuck the edges down around the apples slightly.
Place in the preheated oven and bake for 20-25 minutes until the crust is golden brown.
Let sit for 10 minutes and then carefully run a knife around the edges, place a plate over the top, and flip the tarte onto the plate.
Cut into wedges and serve with some creme fraiche if you wish.