Preheat your oven to 180°C (350°F) and line a 9-inch square baking pan with parchment paper, leaving an overhang for easy removal.
Combine the oats, flour, brown sugar, and a pinch of salt in a mixing bowl. Toss to mix evenly.
Pour the melted butter into the dry mixture and stir until the crumbs form and are evenly coated. The mixture should look crumbly and hold together when pressed.
In a separate bowl, gently toss the fresh raspberries with sugar and lemon zest until coated. Spread this vibrant berry mixture evenly over the bottom of the prepared pan.
Sprinkle the crumble mixture evenly over the berries, pressing lightly to ensure it adheres without compacting.
Bake in the oven for 35-40 minutes, until the topping is golden brown and the berries are bubbling at the edges. You’ll hear a gentle crackle as it bakes.
Remove the pan from the oven and let it cool on a wire rack for at least 20 minutes, allowing the bars to set and the juices to thicken.
Using the parchment overhang, lift the entire block out of the pan and place on a cutting board. Slice into squares or rectangles.
Serve the bars slightly warm or at room temperature. Enjoy their crisp top and juicy, tart interior with a dusting of powdered sugar if desired.
