Preheat your oven to 180°C (350°F). Line your baking pan with parchment paper or lightly butter it to prevent sticking.
In a bowl, combine the flour, oats, brown sugar, and a pinch of salt. Add the cold butter cubes and use your fingers or a pastry cutter to work the butter into the dry ingredients until the mixture resembles coarse crumbs. Chill the crumble mixture in the fridge while preparing the filling.
In a separate bowl, toss the raspberries with granulated sugar, lemon zest, and vanilla extract. Gently stir to coat and let sit for a few minutes to enhance the juices.
Press about two-thirds of the crumble mixture evenly into the bottom of your prepared pan, creating a firm crust.
Spread the raspberry filling evenly over the crust, ensuring the berries are distributed uniformly.
Sprinkle the remaining crumble mixture over the top of the berries, gently pressing it down to help it adhere and form a topping.
Bake in the preheated oven for 35-40 minutes, or until the topping turns a deep golden brown and the filling bubbles around the edges.
Remove from the oven and let the bars cool in the pan for at least 20 minutes. This helps them set and makes slicing easier.
Use a sharp knife to cut into squares, wiping the blade clean between slices for neat edges. Serve warm or at room temperature to enjoy the full burst of flavors and textures.
