Lightly coat 9x13in baking dish with nonstick spray
Prepare the topping
Heat 1/2 stick butter, brown sugar, and cinnamon over low heat until combined and sugar is dissolved
Pour into baking dish & lightly spread into a thin layer*It's okay if its not spread perfectly!
Place the apple slices in a single layer on top
Transfer to fridge while preparing the oatmeal mixture
Prepare oatmeal mixture
In a large bowl, combine wet ingredients: apple sauce, eggs, sugar, butter, milk, and vanilla extract
In a medium bowl, combine dry ingredients: flour, dry oats, cinnamon, baking powder, and salt
Add dry ingredients to wet ingredients. Stir until combined.
Bake
Remove baking dish from fridge and add the oat mixture on top of the apple topping
Bake for ~35 minutes - or until toothpick comes out clean
Flip
When finished baking, carefully use a butter knife along all 4 edges of the baked oatmeal to release all sides before flipping
Using oven mitts, place a large cutting board on top of your baking dish. Hold the cutting board and baking dish together tightly and carefully flip - you might need to borrow an extra pair of hands for help! Return the cutting board to the table with the baking dish on top & upside down
Allow the dish to rest upside down on top of the cutting board for a few minutes so the topping will sink into the baked oatmeal
Carefully remove your baking dish and allow baked oatmeal to cool before slicing
Notes
Nutrition facts will vary depending on specific products and amounts used