Preheat the oven to 375 degrees. Spray an 8x8 baking dish with cooking spray.
Whisk the flour, baking powder and salt in a medium bowl.
In your mixer beat the butter, sugar and zest until fluffy on medium speed, about 3 minutes. Add in the egg and beat until combined. Reduce speed to low and beat the milk until incorporated. Add the flour and beat until smooth, scraping down the bowl as needed. The batter will be thick.
Carefully stir in the peaches and then spread in prepared pan.
TOPPING
Mix the granulated sugar, brown sugar, flour, nutmeg, and salt in a small bowl. Work in butter with your fingers (I use gloves) until crumbly. Scatter evenly over the top of the batter.
Bakre until the top is golden brown and a toothpick comes out clean, about 1 hour. Watch towards the end of the bake and cover with foil so it doesn't burn.