Creating Nectarine Crumble Bars is a straightforward process. Follow these steps to make your delicious dessert:

1. Preheat the Oven: Begin by preheating your oven to 350°F (175°C). Grease a 9×13 inch baking pan.
2. Mix Dry Ingredients: In a large bowl, combine the flour, rolled oats, granulated sugar, brown sugar, baking powder, and salt.
3. Add Butter: Pour in the melted butter and mix until the mixture is crumbly and well combined.
4. Reserve Crumble: Set aside about 1 cup of this mixture for the topping.
5. Prepare Fruit: In a separate bowl, toss the sliced nectarines with lemon juice, cinnamon, and cornstarch until well coated.
6. Layer the Crust: Press the remaining crumble mixture into the bottom of the greased baking pan to form the crust.
7. Add Nectarines: Evenly distribute the nectarines over the crust.
8. Top with Crumble: Sprinkle the reserved crumble mixture over the nectarines.
9. Bake: Place in the preheated oven and bake for 35-40 minutes, or until the top is golden brown and the nectarines are bubbly.
10. Cool: Remove from the oven and allow to cool in the pan for at least 30 minutes before slicing.

By following these steps, you’ll create a batch of Nectarine Crumble Bars that is not only delicious but also simple to prepare.