Start by cooking the bacon—I cut it in half lengthwise and then into about 1" wide pieces. Tumble these into a non-stick pan and place over medium-low heat, letting them gradually heat as to render all the fat. Once the pork starts to give off a humble pitter-patter of sizzling fat you can turn it up a bit and cook to a crisp. Remove the bacon from the pan to a paper towel-lined plate and set aside; reserve 2 tablespoons of the bacon fat and chill in the freezer to firm up (about 30 minutes), and save another tablespoon or two of fat for greasing the baking pan.
Once the butter and bacon fat are filled start the crumble. Load the flour and sugars in either a food processor or stand-up mixer fitted with the paddle attachment (or feel free to do by hand or with a pastry cutter). Mix just a bit to combine before cutting in the butter and bacon fat—in a food processor pulse about 10 - 12 times or mix on medium-low to medium in the stand-up mixer until, either way, you're left with something resembling lumpy wet sand. Stir in the oats and cooked bacon and set aside.
Preheat oven to 350°
Peel the apples, halve them, scoop out the cores, and slice each half into about 6 - 8 slices, depending on the size of the apples, so that you have about 1/4" slices. Toss the apples with the rest of the ingredients in a bowl. I usually start with 2 tablespoons of flour and give the apples a few minutes to macerate in the sugar—if they throw off a lot of juice while they sit I'll add in another tablespoon.
Lightly grease a 9" x 12" (or roughly 2-quart) baking dish with bacon fat and tip the apple mix into the dish. Top the apple mixture with evenly with the crumble, squeezing some of the crumble in your fist and break coarsely over the dish for large pieces of crumble topping.
Bake for about 45 minutes, until the apples are soft and bubbly and the crumble is tanned and crisp. Allow it to cool outside of the oven for about 10 minutes before digging in with some good ice cream not far out of reach.
You can make this ahead and reheat, covered, at 300°, for about 15 minutes or even better yet, assemble the whole thing and bake off when you need it.