1. Preheat the oven to 350°F (177°C). Line an 8-inch square baking pan with parchment paper, leaving an overhang on all sides to easily lift the bars out of the pan (makes cutting much easier). Set aside.\
2. Make the crust/topping: In the bowl of a food processor (or a large bowl), combine the flour, 1/2 cup of the oats, brown sugar, cinnamon, and salt and pulse or whisk to combine. Add the cold cubed butter and pulse the food processor or cut in with a pastry cutter until you have pea-sized crumbs. The mixture should hold together when pressed between your fingers; it shouldn’t look dry or overly crumbly, or the crust may not hold together after baking. Transfer 1 cup (about 125g) of the mixture to a small bowl and mix in the remaining 1 Tbsp oats and sliced almonds (if using). This will be the topping; set aside until needed in step 5.
3. Press the remaining crust mixture into an even layer in the prepared baking pan. Bake the crust for 15 minutes; then, remove the crust from the oven and increase the oven temperature to 375°F (191°C).
4. Make the peach filling immediately before using; if you let the filling sit for too long, the peaches will begin to break down and the filling will become too liquid-y. In a large bowl, whisk together the egg, sugar, flour, vanilla, almond extract, and lemon zest until combined. Fold in the peaches.
5. Pour the filling over the hot crust. Sprinkle the reserved topping mixture evenly over the top.
6. Bake for 40–42 minutes or until lightly browned on top. Allow to cool in the pan on a cooling rack for 1 hour, then transfer to the refrigerator and chill for at least 2 hours or up to 2 days. (Cover tightly if chilling longer than a few hours.)
7. Once chilled, lift the bars out of the pan using the overhang on the sides and cut into squares. For neat squares, use a very sharp knife and wipe it clean with a paper towel after each cut.
8. Make the vanilla icing: In a small bowl or liquid measuring cup, whisk together the confectioners’ sugar, cream/milk, and vanilla until smooth and pourable. Drizzle over peach pie bars.
9. Store leftover peach pie bars tightly covered in the refrigerator for up to 5 days.

Recipe source: https://sallysbakingaddiction.com/peach-pie-crumb-bars/