Preheat the oven to 160°C (320°F) conventional heat and line a baking pan with parchment paper.
Add the raspberries and lemon juice to a small saucepan. If you prefer a sweeter filling, you can also add the maple syrup. Heat over medium heat, stirring regularly, until the raspberries soften and break down.
Remove the saucepan from the heat and immediately stir in the chia seeds. Let the mixture sit for a few minutes to thicken into a jam-like filling.
Meanwhile, mash the ripe banana and peanut butter together in a large bowl with a fork until smooth.
Add the dry ingredients: oats, almond flour, and vanilla whey protein powder. Mix everything well until you have a firm crumble dough.
Spread approximately 2/3 of the oat mixture evenly over the bottom of the prepared baking pan and press it down firmly using the back of a spoon.
Spread the raspberry-chia filling evenly over the base
Crumble the remaining 1/3 of the oat mixture over the top to create the crumble layer.
Bake the crumble bars for approximately 35 minutes, or until the top is nicely golden brown.
Allow the bars to cool completely before slicing.
