Step 1: Preheat your oven to 375°F (190°C) and lightly grease a 9x13 inch baking dish.
Step 2: In a large bowl, combine the sliced nectarines, granulated sugar, 1/4 cup all-purpose flour, lemon juice, 1 teaspoon ground cinnamon, and a pinch of salt. Toss gently until the nectarines are evenly coated.
Step 3: Pour the seasoned nectarine mixture into the prepared baking dish, spreading it into an even layer.
Step 4: In a separate medium-sized bowl, combine the rolled oats, 1/2 cup all-purpose flour, light brown sugar, and 1/2 teaspoon ground cinnamon. Whisk these dry ingredients together thoroughly.
Step 5: Add the cold, diced unsalted butter to the dry crumble mixture. Using your fingertips, a pastry blender, or two forks, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.
Step 6: Evenly sprinkle the crumble topping over the nectarine filling in the baking dish, ensuring it's loosely spread.
Step 7: Bake for 35-40 minutes, or until the topping is golden brown and crispy, and the nectarine filling is bubbly and tender. If the topping browns too quickly, tent loosely with foil.
Step 8: Remove from the oven and let cool on a wire rack for at least 15-20 minutes before serving. This allows the filling to set slightly.