Preheat the oven to 350 degrees F.
Line a 9x13" baking dish with aluminum foil and spray with cooking spray. Set aside.
In a large food processor, add the flour, brown sugar, granulated sugar, baking powder, cinnamon, nutmeg, and pulse until combined.
Add the butter, pulsing until crumbs form.
Add the whisked egg, and pulse until a dough forms. If it seems like it's not coming together in the food processor, transfer it to a large bowl and stir together with a spatula or your hands.
Press a little over half of the mixture into the bottom of the prepared baking dish to form a crust.
Bake in the preheated oven for 15 minutes.
Meanwhile, make the nectarine filling.
In a large bowl, stir together the nectarine slices, sugar, and cinnamon.
In a small bowl, whisk together the cornstarch, lemon juice, and vanilla extract until combined, and then pour over the nectarines. Fold with a spatula to combine.
Using a slotted spoon, transfer the nectarines to the baking dish, layering them over the crust. You don't want all of the juices, so use a slotted spoon.
Sprinkle the rest of the crumb topping over the nectarine mixture.
Bake in the preheated oven for 40-45 minutes, until golden brown.
Let cool completely.
Cut into squares, and serve!