Whisk together flour, brown sugar, walnuts, apple pie spice, and salt.
Cut in butter with a pastry blender until well combined (about the size of peas).
Place apple pie filling into a greased skillet and top with 3/4 cup streusel.
Bake at 300 degrees for 45 minutes or until sides begin to bubble.
Take out of oven and let cool slightly before serving.
Notes
If you're making the apple filling then I recommend using Granny Smith Apples. You can find more information on our Homemade Apple Pie Filling.
This dessert can easily be converted to a single serving by using 6 oz. ramekins. Fill with about 1 cup apple pie filling and top with streusel. Bake for 40 minutes instead of 45.
Make sure to preheat your oven to get the most accurate cook time.