Preheat oven to 350°F. Line an 8x8-inch baking dish with parchment paper.

Add the oats, mashed banana, maple syrup, and vanilla extract to a bowl. Mix until well combined.

Zest the lemon and set the zest aside. Add the raspberries and lemon juice to a food processor and pulse until mostly smooth and jam-like in texture.

Press half of the oat mixture into the bottom of the prepared baking dish, making sure it covers the bottom completely.

Spread the raspberry mixture evenly over the oat layer, then sprinkle the lemon zest on top.

Carefully add the remaining oat mixture over the raspberry layer, scooping small amounts at a time to create the top layer. Avoid pressing down, as this can push the raspberry filling toward the top.

Bake for 25 to 30 minutes, or until the top layer is golden and cooked through.

Allow the bars to cool completely before slicing.