Preheat oven to 350°F. Line a 9x13 pan with parchment paper or aluminum foil, leaving some overhang for easy removal, and set aside.
In a large mixing bowl, whisk together the flour, brown sugar, granulated sugar, baking powder, cinnamon, nutmeg, and salt. Add the cold cubed butter and cut it in with a pastry cutter until the mixture is crumbly and starts to come together. Add the egg and stir it in until fully combined (you may also use the pastry cutter to help blend it in).
Press half of the mixture in the prepared baking pan and place the bowl with the other half in the refrigerator. Bake the crust at 350°F for 15 minutes, then set aside to cool for 5 minutes before adding the peach filling.
For the peach filling:
Combine the chopped peaches, sugar, cornstarch, lemon juice, and cinnamon in a large bowl. Mix until the peaches are fully coated.
Spoon the peach filling on top of the baked crust, using a slotted spoon to leave as much excess juice as possible in the bowl. Spread it around into one even layer.
Remove the remaining crumb mixture from the refrigerator and sprinkle it on top of the peach filling. Bake at 350°F for 40-50 minutes or until the top is set and lightly browned and the peach filling is bubbling around the edges.
Remove from the oven and transfer to a wire rack to cool completely. Once cooled, lift the bars out of the pan using the overhang from the parchment paper or foil, slice into bars, and enjoy!
Store crumb bars in an airtight container in the refrigerator for up to 3 days.