Preheat oven to 375°F (190°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang for easy removal.

Prepare the crumble mixture: In a large bowl, whisk together the flour, oats, brown sugar, cinnamon, and salt.

Cut in the cold, cubed butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs. Reserve about 1.5 cups of this mixture for the topping; the rest will be the base.

Press the remaining crumble mixture firmly and evenly into the bottom of the prepared baking pan to form the base layer.

Prepare the filling: In a separate bowl, gently toss the diced nectarines with the granulated sugar, cornstarch, and lemon juice until evenly coated.

Spread the nectarine filling evenly over the base layer in the baking pan.

Sprinkle the reserved crumble mixture evenly over the fruit filling.

Bake for 35 to 40 minutes, or until the topping is golden brown and the filling is bubbly. If the topping browns too quickly, lightly tent with foil.

Let the bars cool completely in the pan on a wire rack (about 2 hours) before lifting them out by the parchment overhang and cutting into 12 bars. Serve at room temperature or slightly warm.