Creating these delicious Apple Pie Bars is simple when you follow these steps:

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking pan or use parchment paper for easy removal.

2. Prepare the Crust: In a large mixing bowl, combine the softened butter, granulated sugar, and vanilla extract. Mix until creamy. Gradually add the flour and salt, mixing until a dough forms.

3. Press the Crust: Press two-thirds of the dough into the bottom of the prepared baking pan evenly. Bake for 15 minutes until lightly golden.

4. Prepare the Filling: In a separate bowl, toss the sliced apples with lemon juice, brown sugar, cinnamon, nutmeg, and cornstarch. Ensure the apples are well-coated.

5. Spread the Filling: Once the crust is baked, remove it from the oven. Layer the apple mixture evenly over the crust.

6. Make the Topping: In a new bowl, mix the rolled oats, brown sugar, flour, cinnamon, and melted butter until crumbly. Sprinkle this topping over the apple layer.

7. Bake: Return the pan to the oven and bake for an additional 25-30 minutes, or until the topping is golden brown and the apples are tender.

8. Cool: Remove the bars from the oven and let them cool in the pan for about 30 minutes. This will help them set and make cutting easier.

9. Slice and Serve: Once cooled, slice the bars into squares or rectangles. They can be served warm or at room temperature.