PREP: Preheat the oven to 350 F / 175 C and line an 8 inch square baking pan with parchment paper.

MIX: In a large bowl, mix together the flour, both sugars and salt. Use a pastry cutter to cut the butter into the flour until it resembles coarse crumbs and is evenly distributed through the mixture.

RESERVE: Save 1 cup / 155 g of the crust mixture, about one third. Save it in the fridge to use for the topping later.

PRESS: Sprinkle the rest of the crust mixture evenly over the base of the prepared pan, starting with the corners and edges for even thickness throughout. Working from the outside in, press down the crust so that it firms up into a single layer. If you like, use the bottom of a measuring cup to press down and level out any finger dimples for the smoothest finish.

BAKE: Place the baking pan in the preheated oven on the middle shelf and bake until lightly golden and just dry on the surface with a slight golden edge and a light puff, about 8 – 10 minutes.

COMBINE: While the crust is par baking, use a microplane or similar to zest the lemon & a lemon squeezer to juice it. Mix together the arrowroot starch, sugar, lemon juice and zest in a medium mixing bowl until it resembles a slightly thickened slurry. Add the blueberries and stir together until they are coated in the sugar mixture.

TOP: Transfer the par baked crust to a trivet and increase the oven temperature to 375 F / 190 C. Spoon the blueberry mixture onto the hot crust, being mindful of the hot pan and your hands! Squeeze together the refrigerated reserved crust mixture so that it clumps up lightly, then sprinkle it across the surface of the fruit. Let the blueberries peek through the crumbles in places.

FINISH: Return the pan to the oven and bake until the blueberries are just bubbling at the edges and the crumble topping is golden brown, about 30 minutes. If you like, sprinkle with a little turbinado sugar while still warm for a lovely bit of crunch.

COOL: Remove the pan from the oven and place on an oven safe cooling rack or trivet. Let the bars cool to room temperature before attempting to remove them from the pan, about 2 – 3 hours.

SERVE: Once cooled, run a spatula around the edge of the bars to loosen them, then use the parchment paper beneath to lift them free of the baking pan. Slice and serve. Transfer the sliced bars to an airtight container if not serving immediately.