Start by creating the apple pie filling. In a medium-sized saucepan over medium heat, melt the butter. When the butter is melted, add the apple slices, the sugar, and the cinnamon. Stir this well to coat all the apples. Cook this mixture for 10 to 15 minutes until the apples are soft when you poke them with a fork. Remove the saucepan from the heat and set it aside as you prepare the French toast mixture.
Spray a 9×13 baking dish with cooking spray.
Tear the French bread into chunks and arrange them in the casserole dish. Set this aside.
In a medium-sized batter bowl, beat the eggs with a whisk until they are bright yellow and bubbly.
Add the milk, sugar, and vanilla, whisking to combine.
Pour the egg mixture over the bread, pressing the bread into the mixture to allow the bread to soak up the egg mixture.
Spoon the apple pie filling over the top of the French bread and egg mixture. Do not stir, just allow the apples to be in an even layer atop the bread.
Now you can make the crispy topping. You can use the same bowl that you used for the egg mixture. You probably want to wipe out the bowl with a paper towel before you use it. Combine the oats, flour, brown sugar, cinnamon and the melted butter, using a fork to combine them. This should create a crumbly mixture that you can then scatter over the top of the apples in the dish. Create an even layer of crumbs on the top of the casserole.
Cover the dish with plastic wrap and let this casserole sit in the refrigerator for at least 30 minutes. If you want to make this one day ahead of time, you can refrigerate it overnight, and it will be fine.
Heat the oven to 350 degrees. Take the dish from the fridge and allow it to sit out on the countertop for about 20 or 30 minutes to bring it to room temperature.
Place the casserole in the center of the oven and bake it for 45 to 50 minutes until the crispy coating is golden brown and the casserole is fully cooked.
As the casserole is baking, create the glaze for the top of the casserole.
In a small bowl, whisk together the glaze ingredients.
When the casserole is done, remove it from the oven and allow it to cool for about 15 minutes. Drizzle the top of the casserole with the glaze.
Spoon the casserole into bowls and serve.
Leftovers can be saved in the fridge by covering the casserole dish with plastic wrap and storing it in the refrigerator. You can reheat leftovers by microwaving individual servings, or you can pop the whole casserole dish in a 350-degree oven for about 20 minutes. Use leftover casserole within 3 days.