Apple Cinnamon Crumble Bars

Heat the oven to 180 °C (350 °F) and line a 23 x 33 cm (9 x 13 inch) baking pan with parchment, leaving an overhang on the long sides. Melt the butter in a small saucepan over low heat for 3 to 4 minutes, until just liquid but not browned.

Tip: The parchment overhang is your handle for lifting the whole slab out to cut clean bars.

In a large bowl, stir together 2 1/2 cups of the flour with the oats, brown sugar, 1 tsp cinnamon, baking powder and salt. Pour in the melted butter and stir with a fork until it forms damp clumps ranging from sandy crumbs to marble-sized clusters. Scoop out 1 1/2 cups and set aside for the topping.

Tip the rest of the crumble into the lined pan and press it into a firm, even layer with the bottom of a measuring cup. Bake on the middle rack for 12 to 15 minutes, until the base looks set and dry on top and the edges are just turning pale golden.

Tip: Press hard, right into the corners, or the base crumbles apart when you cut the bars.

Meanwhile, toss the sliced apples in a second bowl with the granulated sugar, the remaining flour, the remaining 1 tsp cinnamon, the lemon zest and 1 tbsp lemon juice until every slice is evenly coated and no dry flour remains in the bottom of the bowl.

Tip: The flour thickens the apple juices so the bars set instead of turning soggy.

Spread the apples over the hot base in an even, overlapping layer, scraping in any juices from the bowl. Scatter the reserved crumble over the top, squeezing a few handfuls in your fist so you get some larger clusters among the crumbs.

Return the pan to the oven and bake for 35 to 40 minutes, until the topping is deep golden, the apples are tender when pierced with the tip of a knife, and thick juices bubble up around the edges.

Set the pan on a wire rack and leave the slab at room temperature for at least 1 hour, until the pan feels barely warm and the apple layer no longer shifts when you tilt it. Lift it out by the parchment, cut into 12 bars with a sharp knife, and serve.

Tip: Cut too soon and the filling slumps out; a fully cooled slab gives neat squares.