Preheat the oven to 350 degrees F. Prepare a 8x8" baking dish with cooking spray or overlapping parchment paper for easy removal.
Make the oat crust. Place 1 cup of the oats in the base of a food processor and process until a fine flour forms. Add in the remaining cup of oats, 1/3 cup brown sugar, 1/4 cup walnuts, 1/2 teaspoon cinnamon and salt and pulse until well combined. Add in the butter and pulse again until the mixture resembles wet sand. If the mixture is too crumbly, add in a tablespoon or two of cold water. Depending on the size of your food processor, you may need to stop and scrape down the sides to help mix.
Remove ¾ cup of the dough and set aside. Press the rest of the dough into the prepared pan.
Make the apple filling. Place the apples, remaining teaspoon cinnamon, 3 tablespoons sugar, lemon juice, vanilla extract, cornstarch and nutmeg into a medium saucepan and cook over medium heat until apples are soft and mixture has come together, about 10 minutes. Let cool slightly then pour on the prepared crust.
Crumble the reserved oatmeal mixture over the apple topping and gently press the oat crumble down with your hands to adhere.
Bake for 35-40 minutes until golden brown then remove and let cool completely for the filling to set.
Slice into 12 bars and serve.
Notes
To serve a la' mode. Make the salted caramel topping by microwaving together 1/3 cup coconut oil + 1/3 cup 100% pure maple syrup for 1 minute. Remove and whisk in 3 tablespoons almond butter until smooth. Top warm apple pie bars with a scoop of vanilla dairy-free ice cream and drizzle caramel mixture on top.