Preheat oven to 350 degrees. Line a 8x8" baking dish with parchment paper.
Add sliced apples, 1/2 tsp cinnamon, lemon juice, and 1 tbsp maple syrup to a pan and bring to a boil over medium heat. Lower heat to a simmer for about 8-10 minutes. The apples should be tender but not soft.
Whisk 1/2 tsp arrowroot flour with 2 tbsp of cold water. Pour into apple mixture and stir immediately. Bring mixture back to a boil, then remove from heat, stirring occasionally. Set aside.
Mix remaining dry ingredients, then add wet ingredients and mix until combined.
Pour 3/4 of oat mixture into lined baking dish and spread evenly. Top with apple slices, then add spoonfuls of remaining oat mixture on top of apples. Bake for 30-35 minutes or until top begins to turn golden brown.
Allow bars to cool for at least 10 minutes, then remove to a cutting board using parchment paper. Slice into 9 pieces.
Top with peanut butter or maple syrup before serving, if desired.
Notes
If you'd prefer to use whey protein instead of plant-based, reduce the amount of almond milk you use by half.