Preheat the oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving some overhang for easy removal.
In a bowl, combine the flour, sugar, and salt. Stir in the melted butter and vanilla until a soft dough forms.
Press the dough evenly into the bottom of the prepared pan. Bake for 15–18 minutes, until lightly golden around the edges.
Meanwhile, combine the sliced apples, brown sugar, flour, cinnamon, nutmeg, lemon juice, vanilla, and salt. Toss until the apples are evenly coated.
Spread the apple mixture evenly over the warm crust.
In a separate bowl, mix the flour, oats, brown sugar, cinnamon, salt, melted butter, and chopped pecans until crumbly.
Sprinkle the crumb topping evenly over the apples.
Bake for 40–45 minutes, until the apples are tender and the topping is golden and crisp.
Let the bars cool completely in the pan for at least 1 hour before slicing. For cleaner cuts, chill for 30 minutes after cooling.
Lift the bars out using the parchment overhang and cut into squares. Serve plain or with a scoop of vanilla ice cream.
