Buttery brown sugar–oat crust and crumble cradle a juicy, cinnamon-kissed fresh peach filling, finished with a delicate drizzle of vanilla icing. Each bite balances tender fruit with crisp, golden edges for a bakery-worthy summer bar.
Preheat oven to 350°F (177°C). Line an 8-inch square baking pan with parchment paper, leaving overhang for easy lifting; set aside.
Make the crust/topping base: In a food processor or large bowl, combine flour, 1/2 cup oats, brown sugar, cinnamon, and salt. Pulse or whisk to combine.
Cut in the butter: Add cold, cubed butter and pulse or cut in with a pastry cutter until coarse, pea-sized crumbs form.
Reserve topping: Transfer 1 cup (about 125g) of the crumb mixture to a small bowl; mix in remaining 1 Tbsp oats and sliced almonds (if using). Set aside for topping.
Form and pre-bake crust: Press remaining mixture firmly and evenly into prepared pan. Bake 15 minutes. Remove from oven and increase oven temperature to 375°F (191°C).
Make filling immediately: In a large bowl whisk egg, granulated sugar, flour, vanilla, almond extract, and lemon zest until combined. Fold in chopped peaches.
Assemble: Pour peach filling over the hot crust and spread evenly. Sprinkle reserved crumb topping evenly over peaches.
Bake: Bake at 375°F (191°C) for 35–40 minutes, or until the top is lightly browned.
Cool and set: Cool completely in the pan on a rack, then refrigerate for about 2 hours until fully set and easier to slice.
Make icing: Whisk confectioners’ sugar, cream or milk, and vanilla until smooth and drizzleable.
Finish and serve: Lift bars out using parchment, cut into squares, and drizzle with vanilla icing before serving.