1. Preheat oven to 350°F (175°C) and line a 9×13-inch pan with parchment paper, leaving an overhang for easy removal.

2. In a bowl, mix cake mix, melted butter, and egg until a moist dough forms; press two thirds of the dough into the pan to form the crust.

3. Toss chopped apples with sugar, cinnamon, lemon juice, and flour until evenly coated, then spread over the crust.

4. Combine oats, brown sugar, cinnamon, cold butter, and reserved dough to form coarse crumbs; stir in nuts if using.

5. Sprinkle the streusel evenly over the apples, pressing gently to adhere.

6. Bake for 35 minutes or until the topping is golden and the filling bubbles at the edges.

7. Cool completely in the pan on a wire rack, at least 30 minutes, then lift out using the parchment overhang and cut into squares.

8. Serve warm or chilled, optionally with ice cream or whipped cream.