Prepare Your Baking Tin and Oven: Preheat your oven to 180°C (160°C fan/Gas Mark 4). Grease and line a 23x33cm (9x13 inch) baking tin with parchment paper, leaving an overhang on the longer sides to help lift the bars out later. The kitchen will start to warm up, and you might catch the faint, promising scent of a baking project beginning.

Make the Crumble Dough: In a large bowl, combine the 300g plain flour, 150g caster sugar, baking powder, and salt. Add the chilled, cubed unsalted butter. Using your fingertips or a pastry blender, work the butter into the dry ingredients until the mixture resembles coarse breadcrumbs with some pea-sized butter pieces still visible. This process should take about 5-7 minutes; you'll feel the texture change from dry flour to a sandy, slightly clumpy consistency.

Form the Base Layer: Take about two-thirds of the crumble mixture and press it firmly and evenly into the bottom of your prepared baking tin. Use the back of a spoon or your knuckles to compact it well. It should form a dense, uniform layer. Pop this into the preheated oven to bake for 15-18 minutes, or until the edges begin to turn a very pale golden colour and a subtle buttery aroma fills your kitchen.

Prepare the Strawberry Filling: While the base bakes, combine the hulled and quartered strawberries, 50g granulated sugar, 2 tbsp cornflour, and 1 tbsp lemon juice in a medium bowl. Toss gently until the strawberries are evenly coated and a glossy, slightly thickened syrup begins to form around them. The vibrant red of the strawberries will be beautifully highlighted by the clear, sugary coating.

Assemble the Layers: Remove the partially baked base from the oven. Carefully and evenly spread the strawberry mixture over the warm base, ensuring the fruit is distributed from edge to edge. Over the strawberries, sprinkle the remaining one-third of the crumble mixture. You can gently press it down if you like, but leaving it looser will give a more rustic, crumbly top. At this point, I sometimes brush the top with a little milk or cream for an extra golden finish – it makes a noticeable difference to the final appearance.

Bake Until Golden: Return the baking tin to the oven and bake for another 30-35 minutes, or until the crumble topping is a rich golden brown, the strawberry filling is visibly bubbling around the edges, and your entire home smells wonderfully of sweet berries and toasted butter. You might even hear a faint, gentle sizzling sound from the fruit as it cooks down. Once baked, let the bars cool completely in the tin on a wire rack before attempting to cut them. This cooling time is crucial for the bars to set properly; patience is key here!