Preheat your oven to 180°C (350°F). Grease your 9-inch square baking pan or line it with parchment paper to keep things tidy.
In a mixing bowl, combine the flour, oats, brown sugar, and a pinch of salt. Mix these dry ingredients until well blended, creating a textured base for your crumble.
Add the cold butter pieces to the dry mixture. Use a pastry cutter or forks to cut the butter into the flour until the mixture resembles coarse crumbs. This gives the crust and topping their crumbly texture.
Press about two-thirds of this crumbly mixture evenly into the bottom of your prepared pan, creating a firm, even layer. Bake for 15 minutes until lightly golden and fragrant.
While the crust bakes, toss the raspberries with granulated sugar and lemon zest in a bowl. If you're using jam, swirl a spoonful or two into the berries for extra ooze and sweetness.
Remove the crust from the oven once it’s lightly golden. Spread the raspberry mixture evenly over the warm crust, making sure to cover the entire surface.
Crumble the remaining reserved crumbs evenly over the berry layer, gently pressing some parts into the filling for a rustic appearance.
Bake the assembled bars for another 25–30 minutes until the topping is golden and crispy, and the filling is bubbling around the edges.
Remove from the oven and let the bars cool in the pan for at least 20 minutes. This helps the filling set and makes slicing easier, revealing that luscious, oozing berry center.
Once cooled, cut into squares or rectangles. Serve warm or at room temperature, perhaps with a dollop of whipped cream or a dusting of powdered sugar for extra charm.
