In a large bowl, combine the diced apples, lemon juice, sugar, cinnamon, and a pinch of salt. Toss until evenly coated.

Place the apple mixture in a medium-large pot over medium-high heat. Stir occasionally as the apples cook and release their juices, bringing the mixture to a slow boil.

In a small bowl, mix the cornstarch with cold water to create a slurry.

Once the apples have slightly softened, add the slurry to the pot, stirring constantly. The mixture should thicken within 30 seconds. If it doesn’t, you may need to add a bit more slurry.

Remove from heat and transfer the apple mixture to a container. Allow it to cool completely before using. The filling can be stored in the refrigerator in an airtight container for up to 1 week.

Preheat your oven to 350°F. Line a baking sheet with parchment paper and set aside.

In a medium bowl, combine the brown sugar, flour, butter, cinnamon, and salt. Mix with a wooden spoon until the mixture forms clumps resembling sand.

Spread the streusel mixture evenly on the prepared baking sheet.

Bake for 8-10 minutes, stirring halfway through. The streusel should be golden brown. Allow it to cool completely, then crumble into smaller pieces if necessary.

Make sure the apple pie filling and streusel are completely cool

In a large bowl, combine the cold milk and sweetened condensed milk and whisk together. The mixture will look a little clumpy at first -- don't panic, that's totally normal. Sprinkle the vanilla pudding mix over the top and whisk until smooth and no lumps remain, about 2-3 minutes. I prefer a hand whisk over a hand mixer here because a hand mixer can actually make it clumpier. Once it's smooth, set the bowl aside and let it sit for 5-10 minutes to thicken up. It should look noticeably thicker and more pudding-like before you move on. While it sets, go ahead and make the whipped cream.
3.4 ounce package instant vanilla pudding mix,2 cups cold milk,1 can sweetened condensed milk

In the bowl of a stand mixer or with an electric handheld mixer, add the heavy cream and vanilla extract. Beat on medium-high until stiff peaks form -- this means when you lift the beater, the cream holds its shape and doesn't flop over. Keep a close eye on it because it goes from perfect to overbeaten quickly. If it looks slightly curdled, add a splash of cream and mix on low until it comes back together.

Once your whipped cream is ready, use a rubber spatula to gently fold it into the pudding mixture. Go slow here.

Now it's time to assemble. You can do this in individual cups for a cute single-serve presentation, a large 9x13 casserole dish for a crowd, or a trifle bowl if you want to show off those layers. Start with a generous layer of the pudding mixture on the bottom -- enough to fully coat the base.

Then add a single, neat layer of whole shortbread cookies on top of the pudding, fitting them in as snugly as you can without overlapping too much. Spoon about ½ cup of apple pie filling over the cookies, and a generous amount of the streusel if using, and gently spread it into an even layer, going all the way to the edges. Repeat this -- pudding, cookies, apple filling streusel,— until you run out of ingredients, aiming for 2-3 full layers total depending on your dish.

Finish with a final layer of pudding on top that fully covers everything. Make sure to spread it all the way to the edges and seal it against the sides of the dish so the top layer of cookies doesn't dry out as it sits.

Gently press a sheet of plastic wrap directly against the surface of the pudding -- this prevents a skin from forming on top. Refrigerate for at least 6 hours, but overnight is even better. The cookies need that time to soften into the pudding and become almost cake-like in texture, which is what makes this dessert so good.

Right before serving, take your reserved cookies and crush them in your hands or in a zip-lock bag until you have a mix of bigger pieces and fine crumbs -- you want some texture, not just dust. Scatter that generously over the top. Finish with a few big spoonfuls of apple pie filling. Serve cold straight from the fridge.