Preheat the oven to 350 degrees.
Wash, peel, and thinly slice the apples.
Place a 9-12 inch cast iron skillet over medium-low heat. (The size doesn't really matter, but your filling will be thicker or thinner depending on size, and bake time will need adjusted accordingly.) Add the butter to the pan and melt. Stir in the brown sugar, cinnamon, and salt until combined.
Add the apples to the pan and stir to coat. Cook over medium heat until just beginning to soften, about 5 minutes, stirring often. Remove from the heat.
Add all of the topping ingredients to a medium sized bowl and work them together with a pastry cutter. Alternately, add all of the ingredients to the bowl of a food processor and process them for 30-60 seconds.
Sprinkle the topping evenly over the apples.
Bake for 35-45 minutes, depending on the size of your pan, or until the apples are tender and cooked through.