1️⃣ Preheat and prepare the pan:
Preheat your oven to 350°F (175°C) and grease a 9×13-inch baking dish.

2️⃣ Mix the cake batter:
In a large mixing bowl, combine the yellow cake mix, melted butter, water, and eggs. Mix until the batter is smooth and well incorporated.

3️⃣ Layer the first half of the batter:
Spread half of the batter evenly into the greased baking dish.

4️⃣ Add the apple pie filling:
Spoon the apple pie filling over the batter, spreading it gently. Sprinkle cinnamon evenly over the apples.

5️⃣ Prepare the streusel topping:
In a separate bowl, mix together the brown sugar, rolled oats, flour, and chopped nuts (if using) until combined.

6️⃣ Layer the streusel topping:
Evenly sprinkle the oat mixture over the apple pie filling.

7️⃣ Add the remaining batter:
Carefully pour and spread the remaining cake batter over the top of the oat and apple layers. Spread gently to cover everything.

8️⃣ Bake the cake:
Bake for 30–35 minutes, or until a toothpick inserted into the center comes out clean.

9️⃣ Prepare the glaze:
While the cake bakes, whisk together the powdered sugar with milk (adding a little at a time) until you get a smooth, drizzling consistency.

🔟 Cool and drizzle:
Let the cake cool slightly before drizzling the glaze over the top. Allow it to set for a few minutes before serving.