For the crust, preheat the oven to 400°F. Place the butter into a medium-sized bowl. Microwave for about a minute to melt the butter. In a blender or food processor, grind the oats until they resemble fine flour—this may take a few minutes. Pour the oat flour into the bowl with the melted butter, add sugar and salt, and mix until a crumbly dough forms.
Roll out the dough then place into an 8-inch pie plate or tin. Press the dough to cover the entire surface of the pie tin and trim and crimp the edges. Place a piece of parchment paper over top of the crust and place something heavy (like dry beans or another pie plate) on top to weigh the dough down. Bake 15 minutes then remove and cool. Lower oven temperature to 375°F.
For the filling, heat a large skillet over medium-high heat. Add the butter and melt. Once melted, add the apples and sauté about 8 minutes, tossing often to prevent them from browning. Add the sugar, 1/2 cup of the apple cider and the apple cider vinegar. Bring to a bubble. In a small bowl, mix the cornstarch into the remaining 1/2 cup of apple cider with a fork until smooth. Add the cornstarch mixture, cinnamon, ginger and salt to the pan with the apples. Cook until thickened, about 5 minutes. Remove from the heat and cool.
For the topping, place the butter into a medium bowl and microwave to melt. Mix in the brown sugar, almond meal and salt.
Pour the apple filling mixture into the pre-baked pie shell. The filling should come up to just the top of the pie crust. Pour all of the topping over the top of the pie—it will look like a lot but it will sink down a bit when it cooks. Place pie onto a parchment-lined baking sheet in case any filling bubbles over. Bake for 35-40 minutes or until the top is evenly brown.