• Preheat oven to 350°F. Brush softened butter all over Lodge 9x13-inch Casserole Pan. Line pan with parchment paper, allowing parchment to run up the long sides and slightly over the pan. (This will help with unmolding). Set aside.

Topping:

• Place walnuts, brown sugar, oats, cinnamon, and salt in a food processor. Quickly pulse 3 to 5 minutes until walnuts are coarsely chopped. Add cold butter. Pulse another 3 to 5 times until mixture is crumbly. Set aside.

Filling:

• Whisk together brown sugar, granulated sugar, and cinnamon. Set aside.

Cake:

• In the bowl of a stand mixer fitted with a paddle attachment, cream butter and sugar until smooth. With the mixer running on low, add eggs one at a time until fully incorporated. Add vanilla. Mix until well combined. Scrape down sides of bowl as needed.
• In another bowl, whisk together flour, baking powder, baking soda, and salt. Add dry flour mixture in three additions, alternating with milk in two additions.

  Begin and end with the dry mixture. Mix on low speed until just combined. Scrape down the sides and bottom fo the bowl to ensure thorough mixing.
• Toss chopped apples with cinnamon and 1 Tbsp flour. Fold apples into cake batter until evenly distributed.

Assembly:

• Add half of cake batter to prepared Lodge 9x13-inch Casserole Pan. Spread into an even layer. Sprinkle cinnamon sugar filling all over cake batter. Top with remaining half of cake batter. Carefully spread into an even layer. Sprinkle crumble topping all over cake batter.
• Bake for 45-50 minutes until crumble topping is golden brown and toothpick inserted in center of cake comes out clean. Cool cake in pan for at least 15-20 minutes before slicing and serving.