Prepare a pan or dish for the bottom crust. If you're using a 9x9 metal pan, I lightly butter the inside and line the bottom and two opposite sides with parchment paper. These parchment "handles" make it easy to lift the finished peach shortbread out of the pan. If you’re using a dish with contoured sides, something you can easily get a spatula under, I just lightly spray it with Pam. That’s what I did for this batch, and it worked great.
Weigh or measure the flour into a bowl. Add the sugar, salt, and cinnamon, then give it a quick whisk to blend everything together. Pour this flour mixture into the bowl of your food processor. Cube the cold butter and add it on top. Pulse about 10 to 15 times, or until the mixture starts to come together, it will still look a little dry in spots, and that’s okay. Scoop the shortbread mixture into a bowl and gently mix in any loose flour bits by hand.
Preheat the oven to 350°F (175°C or 180°C). Using a little more than half of the shortbread dough, take clumps and scatter them across the bottom of your prepared pan. Press the dough down to form an even crust, filling in any gaps as needed. Try to make the crust as smooth and level as possible. I like to take a fork and gently press light indentations across the surface, don’t go deep! This helps prevent the crust from puffing up during baking. Bake for 13 to 15 minutes, just until the edges begin to turn golden. (This is a par-bake.)
Let the bottom crust cool once it’s out of the oven. Take the peaches out of the fridge and strain off the liquid, try to remove as much as possible using a fine mesh strainer. Spread the peach mixture evenly over the cooled crust. Next, sprinkle the sliced almonds across the top of the peaches. Finally, take small clumps of the reserved shortbread dough and dot them over the top to create a rustic crumble topping.
Bake at 350°F (175°C or 180°C) for 35–40 minutes, or until the top clumps of shortbread are lightly golden brown. Let the peach shortbread cool in the pan for at least 2 hours. If you're short on time, you can cover the pan and place it in the refrigerator to speed up the cooling process. For the best flavor and texture, serve the bars the same day they’re baked, or at most, the next day. These are really delicious with a scoop of vanilla ice cream!