While apple filling cooks, prepare other ingredients. Pre-heat the oven to 350°F and line an 8" x 8" cake pan with parchment.
Mix all items in a bowl. Stir until a thick, wet, crumbly dough forms. Take ½ cup of the crust mixture and place it in a bowl for topping.
Apply the remaining crust mixture evenly to the parchment-lined cake pan. Multiple fork pokes ensure even baking.
Bake 12–16 minutes on the center rack of the preheated oven until the crust is golden-brown and firm. Remove the crust from the oven.
Bake the bars until the crumb topping is golden-brown, 20–25 minutes. Let crumb bars cool for 30 minutes before slicing and serving.