- Make the filling: In large bowl, mix ingredients until well combined. Set aside.
- Make the topping: Preheat oven to 350 degrees. In medium bowl, combine walnuts, flour, oats, brown sugar, and salt. Using a fork or your hands, thoroughly incorporate butter into mixture.
- To assemble: Transfer apple filling to 9-inch-square baking dish. Top evenly with oat mixture and bake until golden and bubbling, about 45 minutes.