Follow these steps carefully to bake the perfect batch of Peach Crumble Bars. Precision in baking often leads to the best results!

1. Preheat and Prepare: Preheat your oven to 375°F (190°C). Grease and flour a 9×13 inch baking pan. For easier removal and cleanup, you can line the pan with parchment paper, leaving an overhang on two opposite sides to use as handles later. Ensure the paper lies flat in the corners.
2. Make the Crumble Mixture: In a large bowl, whisk together the 3 cups of all-purpose flour, 1 ½ cups of granulated sugar, baking powder, ½ teaspoon salt, and ¾ teaspoon ground cinnamon. Ensure these dry ingredients are well combined.
3. Cut in the Butter: Add the cold, cubed butter to the dry ingredients. Using a pastry blender, two knives, or your fingertips, cut the butter into the flour mixture until it resembles coarse crumbs or small peas. Work relatively quickly to keep the butter cold. If using a food processor, pulse carefully until the desired texture is reached, being careful not to over-process into a paste.
4. Divide the Mixture: Measure out approximately 3 cups of this crumb mixture and set it aside in a separate bowl. This will be used for the topping.
5. Add Oats to Topping: To the reserved 3 cups of crumb mixture, stir in the 1 ½ cups of rolled oats. Mix until the oats are evenly distributed. This enhanced mixture is your crumble topping. Keep this bowl aside (or even pop it in the fridge if your kitchen is very warm).
6. Finish the Crust Mixture: To the remaining larger portion of the crumb mixture in the original bowl, add the lightly beaten large egg. Mix with a fork or your hands until the dough just starts to come together and is moistened. It will still be crumbly but should hold together when pressed.
7. Press in the Crust: Transfer the egg-moistened crust mixture to your prepared 9×13 inch baking pan. Press it down firmly and evenly across the bottom of the pan to form the base crust. Using the bottom of a measuring cup or glass can help create a flat, even surface. Ensure the crust goes right to the edges.
8. Prepare the Peach Filling: In a separate medium bowl, gently combine the peeled and sliced/diced peaches, ½ cup granulated sugar, ¼ cup all-purpose flour, lemon juice, ½ teaspoon ground cinnamon, optional nutmeg, and the pinch of salt. Toss carefully until the peaches are evenly coated with the sugar and spice mixture. Let it sit for a minute or two while you proceed.
9. Assemble the Bars: Spoon the peach filling evenly over the pressed crust in the baking pan. Spread it out gently to cover the entire base, reaching the edges.
10. Add the Crumble Topping: Take the reserved oat-crumb mixture (the one you set aside earlier) and sprinkle it evenly over the peach filling. Ensure the entire surface is covered with the delicious crumble. Gently pat it down ever so slightly, but don’t press hard – you want it to remain crumbly.
11. Bake: Place the baking pan in the preheated oven. Bake for 40-50 minutes, or until the crumble topping is golden brown and the peach filling is bubbly around the edges. The exact time can vary depending on your oven and the juiciness of your peaches. Look for visual cues – a beautiful golden top and bubbling juices are key indicators of doneness.
12. Cool Completely: This step is crucial! Remove the pan from the oven and place it on a wire rack. Let the bars cool completely in the pan before cutting. This usually takes at least 2-3 hours, sometimes longer. Cooling allows the filling to set properly, ensuring clean cuts and preventing the bars from falling apart. Attempting to cut them while warm will result in a delicious, but messy, crumble rather than distinct bars.
13. Cut and Serve: Once completely cooled, if you used parchment paper, use the overhangs to lift the entire slab out of the pan onto a cutting board. If not, carefully cut the bars directly in the pan. Use a sharp knife to cut into desired sizes (squares or rectangles – 16-24 bars depending on size). Serve and enjoy!