Preheat oven to 350°F (175°C). Line a 9x9" (23x23cm) baking pan with parchment paper with an overhang on two opposite sides. Set aside.
Make the shortbread crust in a food processor: Place flour, sugar, salt, and butter in a food processor and pulse until it starts to clump. Transfer to the prepared baking pan and press with your fingers into the bottom of the pan. Then place a piece of paper on top of the crust and even with a flat-bottomed cup. Bake for 15 minutes until crust is lightly browned.
If you don't have a food processor, then make the shortbread crust with a pastry cutter or two forks instead: In this case mix together flour, salt, and sugar in a large mixing bowl. Then add butter and cut it into the mixture until it starts to clump.
Apple filling: In a large bowl, stir together apples, flour, sugar, cinnamon, and vanilla just until combined. Set aside.
Make the pecan topping in a food processor: Place pecans, flour, sugar, salt, butter, and oats in a food processor and pulse until it starts to clump. If you don't have a food processor, then make the pecan topping with a pastry cutter or two forks instead: In this case chop pecans finely. Transfer to a large bowl and add flour, sugar, salt, and oats. Then add butter and cut it into the mixture until it starts to clump.
Remove crust from the oven and evenly arrange apples without liquid (if there is any) on top of the baked crust. Sprinkle with the pecan topping. Bake at 350°F/ 175°C for 30-35 minutes. Remove from the oven and let cool completely. Then lift the bars out of the pan with the paper and cut into 9 bars (or more if preferred). Store in an airtight container in the fridge for up to 3 days.