Preheat your oven to 375°F (190°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides to easily lift the bars out later. Lightly grease the parchment paper.

In a large mixing bowl, whisk together the flour, 0.5 cup granulated sugar, and salt for the crumble. Add the cold cubed butter. Using a pastry blender or your fingertips, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining. Don't overmix!

Press about two-thirds of the crumble mixture evenly into the bottom of the prepared baking pan. Reserve the remaining one-third of the crumble for the topping.

Peel and pit your peaches, then slice them into about 0.5-inch thick wedges. You should have about 4-5 cups of sliced peaches. If using frozen peaches, make sure they are thawed and drained well.

In another medium bowl, gently combine the sliced peaches with the remaining 0.33 cup granulated sugar, cornstarch, fresh lemon juice, and ground cinnamon. Toss everything together until the peaches are well coated. The cornstarch will help thicken the juices as they bake.

Pour the peach filling evenly over the pressed crumble base in the baking pan. Sprinkle the reserved one-third of the crumble mixture over the peaches.

Bake for 45-55 minutes, or until the crumble topping is golden brown and the peach filling is bubbly and tender. The edges should look set and slightly caramelized.

This is crucial! Let the bars cool in the pan on a wire rack for at least 2-3 hours, or even better, chill them in the refrigerator for an hour or two. This allows the peach filling to set properly, making them much easier to cut into neat bars. Once cool, lift them out using the parchment paper overhang and slice into squares.