Step 1: Preheat your oven to 375°F (190°C). Line a 9x13 inch baking dish with parchment paper, leaving an overhang on the sides to easily lift the bars out later.
Step 2: In a light-colored saucepan, melt 1 cup unsalted butter over medium heat. Continue to cook, stirring occasionally, until the butter turns a rich golden-brown and nutty aromas fill your kitchen. Immediately remove from heat and pour into a heatproof bowl to stop the cooking process. Let it cool slightly, about 10 minutes.
Step 3: In a large mixing bowl, whisk together 2½ cups all-purpose flour, 1 cup granulated sugar, and ½ teaspoon salt.
Step 4: Divide the slightly cooled brown butter. Reserve ¼ cup for the crumble topping. Add the remaining ¾ cup brown butter to the dry ingredients for the crust. Mix with a fork or your fingers until the mixture resembles coarse crumbs and holds together when pressed.
Step 5: Press about two-thirds of this mixture evenly into the bottom of your prepared 9x13 inch baking dish. Bake for 12-15 minutes, or until lightly golden.
Step 6: While the crust bakes, wash and pit the 6-8 medium nectarines. Slice them into ½-inch thick wedges.
Step 7: In a medium bowl, gently toss the sliced nectarines with ¼ cup granulated sugar, 2 tablespoons all-purpose flour, and ½ teaspoon ground cinnamon.
Step 8: To the remaining one-third of the crust mixture, add the reserved ¼ cup brown butter, ½ cup rolled oats, ¼ cup packed light brown sugar, and ½ teaspoon ground cinnamon. Mix until well combined and crumbly.
Step 9: Once the crust is lightly golden, remove it from the oven. Evenly spread the nectarine filling over the hot crust.
Step 10: Sprinkle the brown butter crumble topping generously over the nectarine layer.
Step 11: Return the baking dish to the oven and bake for another 35-45 minutes, or until the topping is golden brown and the nectarines are tender and bubbling.
Step 12: Let the bars cool completely in the pan on a wire rack before cutting (2-3 hours). Lift the parchment paper to remove the entire block of bars, then slice into squares or rectangles. Store any leftover bars in an airtight container at room temperature for up to 3 days, or in the refrigerator for up to 5 days.