To a large pan, add your butter on medium-high heat until melted. Add your apples and stir them to coat them in the butter. Sprinkle your white sugar, brown sugar, cinnamon, allspice and salt across the top and stir to combined. Lower the heat to medium and cook for 5 minutes, or till the apples start to soften.
Sprinkle the flour and cornstarch across the top and stir together, and cook another 3 minutes.
Remove the pan from the heat, add the vanilla and apple cider vinegar to the apples and stir them in. Let the fruit mixture cool before pouring it into your pie crust.
To a small, microwaveable bowl, add your butter and melt it. Stir in your sugar. Then add your flour and salt and stir them in till all is well combined. Put in the fridge while the pie filling cools.
Preheat your oven to 375.
Pour your apple pie filling into your gluten-free pie crust. With your hands, sprinkle the crumb topping across the top (breaking up any large clumps).
Bake in the oven for 50-60 minutes, or till the fruit is bubbling on top. Check to make sure the crumb topping isn't burning (if it is, cover the pie with a loose piece of aluminum foil). Let cool entirely (or till just warm) prior to slicing and serving, to let the fruit set.
Pie can be covered and kept room temperature for 1-2 days, or stored in the fridge for about a week.