Preheat oven to 400 degrees
Press the pie crust into a 9 inch pie plate, trimming off the excess and crimping the edges. Put in the fridge.
1 sheet frozen pie crust
In a medium size mixing bowl, make the crumble topping by combining 1 cup rolled oats, 1/2 cup packed brown sugar and 6 tablespoons all-purpose flour. Toss the ingredients together to evenly combine, breaking up any chunks of brown sugar. Combine the cold butter into the dry ingredients by squeezing and breaking it up with your fingers until crumbs form. Cover with saran wrap and put in the freezer.
1 cup rolled oats, 1/2 cup packed brown sugar, 6 tablespoons all-purpose flour, 8 tablespoons cold butter cut into small pieces
Peel and core the granny smith apples. Slice the apples into slices that are 1/8" thick and 1" long. In a large mixing bowl, toss the apple slices with the lemon zest and lemon juice. Then add in the 1/2 cup granulated sugar, 1/4 brown sugar, 2 tablespoons all-purpose flour, cinnamon and nutmeg. Toss to evenly combine.
5 large granny smith apples, Zest of 1 lemon, Juice of half a lemon, 1/2 cup granulated sugar, 1/4 cup packed brown sugar, 2 tablespoons all-purpose flour, 1 - 1/2 teaspoons cinnamon, 1/2 teaspoon nutmeg
Take the pie crust out of the fridge and pour in the apple mixture. The apples should be about 1/4" - 1/2" taller than the edge of the pie pan.
Take the crumble topping out of the freezer. Should be very cold and almost hard. This is good. Take a knife or pastry cutter and break up into large pieces. Then, use your fingers to break into smaller crumb pieces, using medium pressure to press into an even layer to the pie.
Place the pie plate on a parchment lined baking sheet to protect your oven if it bubbles over. Bake at 400ºF degrees for 20 minutes, then lower the oven to 350ºF degrees and bake for another 35-40 minutes, or until the crumb topping is golden and the pie is bubbling.
Allow to cool for at least 6 hours at room temperature uncovered. I like to make mine 24 hours in advance, and leave it uncovered on the counter.
Serve at room temperature or reheated in the microwave for 45 seconds. Serve with ice cream or whipped cream.
Refrigerate after 24 hours. Will keep in the fridge for 1 week.
See Recipe Notes below for more tips, tricks and recipe substitutions!