Preheat and Prepare: First, preheat your oven to 180°C (160°C fan) / Gas Mark 4. Grease and line a 20cm (8-inch) square baking tin with baking parchment, leaving some overhang on the sides to make it easier to lift the bars out later.

Make the Crumble Mixture: In a large mixing bowl, whisk together the plain flour, rolled oats, light brown sugar, bicarbonate of soda, and salt. Add the cold, cubed butter.

Rub in the Butter: Using your fingertips, rub the butter into the dry ingredients until the mixture resembles coarse breadcrumbs with some larger, pea-sized lumps remaining. I find that this hands-on method gives the best texture, but you can also pulse it a few times in a food processor.

Form the Base: Take out approximately two-thirds of the crumble mixture (about 550g) and press it firmly and evenly into the bottom of your prepared tin. I use the back of a spoon or the bottom of a glass to get a nice, compact layer. This ensures the base is sturdy.

Bake the Base: Bake the base for 15 minutes until it's lightly golden around the edges. This pre-baking step is crucial for preventing a soggy bottom.

Prepare the Filling: While the base is baking, make the filling. In a separate bowl, gently toss the blueberries, granulated sugar, cornflour, lemon juice, and vanilla extract together until the berries are evenly coated.

Assemble the Bars: Once the base is out of the oven, carefully spoon the blueberry filling over the top and spread it into an even layer, leaving a small border around the edge.

Add the Topping and Bake: Sprinkle the remaining one-third of the crumble mixture evenly over the blueberry filling. Place the tin back in the oven and bake for a further 25-30 minutes, or until the crumble topping is golden brown and the fruit filling is bubbling thickly at the edges.

Cool Completely: This is the hardest part! You must let the bars cool completely in the tin before attempting to slice them. I usually leave them on a wire rack for at least 2 hours. Cooling fully allows them to set properly, ensuring clean, neat slices.