Preheat your oven to 350°F (175°C). Lightly grease a 9x13 inch baking dish or a 10-inch cast iron skillet with butter.
Peel, core, and slice the apples into ¼-inch thick slices, ensuring uniform thickness for even cooking.
In a large bowl, gently toss the sliced apples with bourbon, brown sugar, lemon juice, vanilla extract, cinnamon, nutmeg, salt, and cornstarch.
In a dry skillet over medium heat, toast the chopped pecans for 5-7 minutes, stirring occasionally until fragrant.
In a medium bowl, combine oats, flour, both sugars, toasted pecans, cinnamon, and salt. Add the cold, cubed butter and use your fingers or a pastry cutter to work the mixture until it resembles coarse crumbs with some pea-sized pieces of butter remaining.
Drizzle the bourbon over the topping and mix gently to incorporate.
Transfer the apple mixture to your prepared baking dish, ensuring an even layer. Sprinkle the pecan crisp topping evenly over the apples, covering them completely. Avoid pressing down on the topping.
Bake in the preheated oven for 45-50 minutes, or until the topping is golden brown and the filling is bubbling around the edges. If the topping begins to brown too quickly, loosely cover with aluminum foil.
Remove from oven and let rest for 15 minutes before serving to allow the juices to thicken.