1. Prepare the Crust

• Preheat Oven: Preheat your oven to 350°F (175°C).
• Mix Dry Ingredients: In a large bowl, whisk together the flour, sugar, and salt.
• Cut in Butter: Add the cold, cubed butter to the flour mixture. Use a pastry cutter or your fingers to combine until it resembles coarse crumbs.
• Add Egg and Vanilla: Mix in the egg and vanilla extract until the dough forms.
• Press into Pan: Grease a 9×13-inch baking dish. Press two-thirds of the dough evenly into the bottom of the pan. Reserve the remaining dough for the topping.

2. Prepare the Apple Filling

• Slice Apples: Peel and thinly slice the apples.
• Mix Filling Ingredients: In a large bowl, toss the apples with brown sugar, lemon juice, cinnamon, nutmeg, cornstarch, and a pinch of salt until well coated.
• Spread Filling Over Crust: Evenly distribute the apple mixture over the crust in the baking dish.

3. Prepare the Topping

• Mix Topping Ingredients: In another bowl, combine oats, flour, brown sugar, and cinnamon. Drizzle in the melted butter and stir until crumbly.
• Sprinkle on Top: Evenly sprinkle the topping over the apple filling.

4. Bake the Bars

• Bake: Place the baking dish in the preheated oven and bake for 45–50 minutes, or until the apples are bubbly and the topping is golden brown.
• Cool: Remove from the oven and let the bars cool in the pan for at least 20 minutes before cutting.