Gather the ingredients. Preheat the oven to 425 degrees F (220 degrees C). Peel and core apples, then thinly slice. Set aside.
Melt 1 stick (0.5 cups) of butter in a saucepan over medium heat. Add 3 tablespoons of flour and stir to form a paste; cook until fragrant, about 1 to 2 minutes. Add brown sugar, 0.5 cups white sugar, cinnamon to taste, and water; bring to a boil. Reduce the heat to low and simmer for 3 to 5 minutes. Add apples and stir until each piece is coated. Remove from the heat.
Pour apple mixture into the pie crust, making a small mound.
Make crumbles by mixing the remaining stick (0.5 cups) of butter with the remaining flour and sugar. Knead well.
TIP: do not add all the flour and sugar at once. Add a little bit of both at first, then gradually add more. The measurements are not accurate; try some crumbs throughout to see if you want more sugar or not. If big chunks stick together, the crumbles are not dry enough and you will need more flour and/or sugar. You can also add a little bit of ground cinnamon to the crumbles.
Bake in the preheated oven for 15 minutes. Reduce the temperature to 350 degrees F (175 degrees C) and continue baking until apples are soft, 35 to 45 minutes.
Serve (maybe with vanilla ice cream?) and enjoy!
