In a small mixing bowl, combine 1 1/2 tbsp ground flaxseed with 3 tbsp warm water and set aside for about 5 minutes. This is you flax egg.
In a large mixing bowl, add the flour, oats, sugar, baking soda, and salt. Mix until evenly combined. Add the oil and flax egg and mix until a dough has formed.
Press the dough into the bottom of a parchment-lined baking dish until flat and even throughout.
Bake the base at 350 degrees Fahrenheit for 8-10 minutes. Remove from the oven and set aside.
Make the Crumb Topping
To make the crumble topping, just add all ingredients in a medium mixing bowl and combine with your fingers until a dry, crumbly texture is achieved.
Set the crumble topping aside.
Make the filling
Peel and dice your apples to your desired size.
Add the diced apples, spices, and cornstarch together in a cold pan. Mix until the apples are evenly coated. Add the water and lemon juice and heat the mixture over medium heat, stirring constantly.
Add more water as necessary to prevent the filling sticking to the bottom or sides of the pan.
Once the filling has thickened, remove it from heat and set it aside.
Assemble the Bars
Pour the apple pie filling over the pre-baked base and spread it into an even layer.
Add the crumble topping to your desired thickness.
Return the bars to the oven and bake 20-30 minutes at 350 F or until the crumble topping is slightly golden brown.
Let cool slightly, use the edges of the parchment paper to lift the bars out of the baking dish to slice and serve.