Step 1: Preheat your oven to 350°F (175°C). Line a 9×13-inch baking dish with aluminum foil, leaving an overhang on the sides, and lightly coat it with cooking spray.
Step 2: In a food processor, combine the flour, brown sugar, 1/4 cup granulated sugar, baking powder, 1/2 teaspoon cinnamon, nutmeg, and salt. Pulse until everything is well-mixed.
Step 3: Add the cold, cubed butter and pulse until the mixture forms coarse crumbs.
Step 4: Add the whisked egg and pulse until a cohesive dough forms. If needed, transfer the mixture to a bowl and mix with your hands or a spatula until fully combined.
Step 5: Press a little over half of the dough mixture evenly into the bottom of the prepared baking dish to form the crust.
Step 6: Bake the crust in the preheated oven for 15 minutes, or until lightly golden.
Step 7: While the crust bakes, prepare the nectarine filling. In a large bowl, gently toss the sliced nectarines with the remaining 1/4 cup granulated sugar and 1/2 teaspoon cinnamon.
Step 8: In a separate small bowl, whisk together the cornstarch, lemon juice, and vanilla extract until smooth. Pour this mixture over the nectarines and stir to coat evenly.
Step 9: Using a slotted spoon, carefully transfer the nectarine mixture onto the baked crust, spreading it evenly and leaving behind any excess juice.
Step 10: Sprinkle the remaining crumble topping mixture evenly over the nectarine filling.
Step 11: Bake for an additional 40-45 minutes, or until the crumble topping is golden brown and the filling is bubbly around the edges.
Step 12: Allow the bars to cool completely in the pan on a wire rack before lifting them out by the foil overhang and cutting them into squares for serving. Cooling completely is essential for neat slices.