Preheat your oven to 375 degrees F. Add the apples to a large mixing bowl and set it aside.
In a medium mixing bowl, combine the ingredients - flour, dark brown sugar, lemon zest, cinnamon, nutmeg, and salt. Pour the cinnamon mixture over the apple and toss to combine. Set aside to macerate while you cook the bacon.
Place a 12-inch cast iron skillet over medium heat. (see note) Cook the bacon pieces until crispy, about 10 minutes. Transfer the bacon to a paper towel-lined plate to drain. Pour off all but 1 tablespoon of bacon grease from the skillet.
Pour the apples into the skillet and spread into an even later. Set aside.
In a medium bowl, mix together the instant oats, flour, sugar, and dark brown sugar to make the topping. Cut in the cold butter with a fork or pastry cutter until the mixture has a sand-like texture (like a pie crust).
Mix half of the cooked bacon into the streusel topping and then sprinkle it all evenly over the apples.
Bake for 40-45 minutes or until the filling is bubbling, the apples are tender, and the topping is golden brown. Remove from the oven and let cool for 10 minutes.
Portion the apple crisp into bowls and sprinkle with the reserved bacon. Top with vanilla ice cream and/or bourbon caramel sauce if desired. Serve warm.
Notes
If you don't have a cast iron skillet available, you can use a 9-inch x 13-inch baking dish to bake the apple crisp. Cook the bacon in a skillet and pour 1 tablespoon of bacon grease into the baking dish before spreading it around the pan. Then proceed as directed.
If you don't have salted butter on hand, add 1/4 teaspoon salt to the topping mixture.