• Preheat your oven to 350°F (175°C). Generously grease a 9×13-inch casserole pan with softened butter, then line it with parchment paper. Allow the parchment to hang slightly over the longer edges for easy lifting once baked. Set aside.

To Make the Crumble Topping:

• In a food processor, combine the toasted walnuts, brown sugar, rolled oats, cinnamon, and salt. Pulse 3 to 5 times until the walnuts are roughly chopped and everything is well mixed. Add in the cold butter and pulse a few more times until the mixture resembles a coarse crumble. Set aside while you prepare the rest.

To Make the Filling:

• In a small bowl, mix together the brown sugar, granulated sugar, and cinnamon until fully combined. Set aside.

To Prepare the Cake Batter:

• In the bowl of a stand mixer fitted with a paddle attachment, beat the butter and granulated sugar on medium speed until smooth and fluffy. With the mixer running on low, add the eggs one by one, ensuring each is incorporated before adding the next. Mix in the vanilla extract and continue to beat until everything is blended, scraping down the sides of the bowl as needed.
• In a separate mixing bowl, whisk together the flour, baking powder, baking soda, and salt. Add the dry ingredients in three batches, alternating with the milk in two additions. Always begin and finish with the dry mixture. Mix just until everything is combined—do not overmix. Scrape down the sides and bottom of the bowl to ensure the batter is uniform.
• Toss the chopped Fuji apples with the flour and cinnamon until evenly coated. Gently fold the apple mixture into the batter, making sure the fruit is well distributed throughout.

To Assemble the Cake:

• Pour half of the cake batter into the prepared pan and smooth it into an even layer with an offset spatula. Evenly sprinkle the cinnamon-sugar filling over the batter. Spoon the remaining batter on top and spread it carefully to cover the filling layer. Finish by scattering the crumble topping across the surface of the batter.
• Bake for 45 to 50 minutes, or until the topping is deeply golden and a toothpick inserted in the center comes out clean. Allow the cake to cool in the pan for 15 to 20 minutes before slicing into generous squares and serving warm or at room temperature.